ci: add caching actions

Add "Restore" and "Save" caching actions.

These actions reduce boilerplate in the main ci.yml configuration file.

These actions are implemented so that caches will be saved on `push`
only.

When a pull request is opened it will cache hit on the caches from the
lastest push, or in the case of depends will hit on any matching depends
hash, falling back to partial matches.

Depends caches are hashed using
`$(git ls-tree HEAD depends "ci/test/$FILE_ENV" | sha256sum | cut -d' ' -f1)`
and this hash is passed in as an input to the actions. This means we
direct cache hit in cases where depends would not be re-built, otherwise
falling back to a partial match.

Previous releases cache is hashed similarly to depends, but using the
test/get_previous_releases.py file.

The cirruslabs cache action will fallback transparently to GitHub's
cache in the case that the job is not being run on a Cirrus Runner,
making these compatible with running on forks (on free GH hardware).

.github/actions/restore-caches/action.yml

@@ -0,0 +1,47 @@
+name: 'Restore Caches'
+description: 'Restore ccache, depends sources, and built depends caches'
+runs:
+  using: 'composite'
+  steps:
+    - name: Restore Ccache cache
+      id: ccache-cache
+      uses: cirruslabs/cache/restore@v4
+      with:
+        path: ${{ env.CCACHE_DIR }}
+        key: ccache-${{ env.CONTAINER_NAME }}-${{ github.run_id }}
+        restore-keys: |
+          ccache-${{ env.CONTAINER_NAME }}-
+
+    - name: Restore depends sources cache
+      id: depends-sources
+      uses: cirruslabs/cache/restore@v4
+      with:
+        path: ${{ env.SOURCES_PATH }}
+        key: depends-sources-${{ env.CONTAINER_NAME }}-${{ env.DEPENDS_HASH }}
+        restore-keys: |
+          depends-sources-${{ env.CONTAINER_NAME }}-
+
+    - name: Restore built depends cache
+      id: depends-built
+      uses: cirruslabs/cache/restore@v4
+      with:
+        path: ${{ env.BASE_CACHE }}
+        key: depends-built-${{ env.CONTAINER_NAME }}-${{ env.DEPENDS_HASH }}
+        restore-keys: |
+          depends-built-${{ env.CONTAINER_NAME }}-
+
+    - name: Restore previous releases cache
+      id: previous-releases
+      uses: cirruslabs/cache/restore@v4
+      with:
+        path: ${{ env.PREVIOUS_RELEASES_DIR }}
+        key: previous-releases-${{ env.CONTAINER_NAME }}-${{ env.PREVIOUS_RELEASES_HASH }}
+        restore-keys: |
+          previous-releases-${{ env.CONTAINER_NAME }}-
+
+    - name: export cache hits
+      shell: bash
+      run: |
+        echo "depends-sources-cache-hit=${{ steps.depends-sources.outputs.cache-hit }}" >> $GITHUB_ENV
+        echo "depends-built-cache-hit=${{ steps.depends-built.outputs.cache-hit }}" >> $GITHUB_ENV
+        echo "previous-releases-cache-hit=${{ steps.previous-releases.outputs.cache-hit }}" >> $GITHUB_ENV

.github/actions/save-caches/action.yml

@@ -0,0 +1,39 @@
+name: 'Save Caches'
+description: 'Save ccache, depends sources, and built depends caches'
+runs:
+  using: 'composite'
+  steps:
+    - name: debug cache hit inputs
+      shell: bash
+      run: |
+        echo "depends sources direct cache hit to primary key: ${{ env.depends-sources-cache-hit }}"
+        echo "depends built direct cache hit to primary key: ${{ env.depends-built-cache-hit }}"
+        echo "previous releases direct cache hit to primary key: ${{ env.previous-releases-cache-hit }}"
+
+    - name: Save Ccache cache
+      uses: cirruslabs/cache/save@v4
+      if: ${{ (github.event_name == 'push') && (github.ref_name == github.event.repository.default_branch) }}
+      with:
+        path: ${{ env.CCACHE_DIR }}
+        key: ccache-${{ env.CONTAINER_NAME }}-${{ github.run_id }}
+
+    - name: Save depends sources cache
+      uses: cirruslabs/cache/save@v4
+      if: ${{ (github.event_name == 'push') && (github.ref_name == github.event.repository.default_branch) && (env.depends-sources-cache-hit != 'true') }}
+      with:
+        path: ${{ env.SOURCES_PATH }}
+        key: depends-sources-${{ env.CONTAINER_NAME }}-${{ env.DEPENDS_HASH }}
+
+    - name: Save built depends cache
+      uses: cirruslabs/cache/save@v4
+      if: ${{ (github.event_name == 'push') && (github.ref_name == github.event.repository.default_branch) && (env.depends-built-cache-hit != 'true' )}}
+      with:
+        path: ${{ env.BASE_CACHE }}
+        key: depends-built-${{ env.CONTAINER_NAME }}-${{ env.DEPENDS_HASH }}
+
+    - name: Save previous releases cache
+      uses: cirruslabs/cache/save@v4
+      if: ${{ (github.event_name == 'push') && (github.ref_name == github.event.repository.default_branch) && (env.previous-releases-cache-hit != 'true' )}}
+      with:
+        path: ${{ env.PREVIOUS_RELEASES_DIR }}
+        key: previous-releases-${{ env.CONTAINER_NAME }}-${{ env.PREVIOUS_RELEASES_HASH }}