ci: add job to determine runner type

To remove multiple occurances of the respository name, against which we
compare `${{ github.repository }}` to check if we should use Cirrus
Runners, introduce a helper job which can check a single environment
variable and output this as an input to subsequent jobs.

Forks can maintain a trivial patch of their repo name against the
`REPO_USE_CIRRUS_RUNNERS` variable in ci.yml if they have Cirrus Runners
of their own, which will then enable cache actions and docker build
cache to use Cirrus Cache.

It's not possible to use `${{ env.USE_CIRRUS_RUNNERS }}` in the
`runs-on:` directive as the context is not supported by GitHub.

If it was, this job would no longer be necessary.

.github/workflows/ci.yml

@@ -30,6 +30,22 @@ defaults:
     shell: bash
 
 jobs:
+  runners:
+    name: 'determine runners'
+    runs-on: ubuntu-latest
+    outputs:
+      use-cirrus-runners: ${{ steps.runners.outputs.use-cirrus-runners }}
+    steps:
+      - id: runners
+        run: |
+          if [[ "${REPO_USE_CIRRUS_RUNNERS}" == "${{ github.repository }}" ]]; then
+            echo "use-cirrus-runners=true" >> "$GITHUB_OUTPUT"
+            echo "::notice title=Runner Selection::Using Cirrus Runners"
+          else
+            echo "use-cirrus-runners=false" >> "$GITHUB_OUTPUT"
+            echo "::notice title=Runner Selection::Using GitHub-hosted runners"
+          fi
+
   test-each-commit:
     name: 'test each commit'
     runs-on: ubuntu-24.04