ci: run the heavy Gradle jobs on the self-hosted runners

Requires the Android SDK in the runner image. Also gates the disk-cleanup
step to hosted runners, where it would otherwise strip the host.

.github/workflows/build.yml

@@ -21,7 +21,7 @@ env:
 jobs:
   build:
     name: Build APKs
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, Linux, X64]
     permissions:
       # A job-level block replaces the workflow-level one, so contents:read has to be
       # repeated here or checkout fails on this private fork.
@@ -38,6 +38,9 @@ jobs:
       cancel-in-progress: true
     steps:
       - name: Free Disk Space (Ubuntu)
+        # Only meaningful on GitHub's throwaway VMs. On a persistent self-hosted runner this
+        # would delete dotnet/haskell/docker images from the host on every run.
+        if: runner.environment == 'github-hosted'
         uses: jlumbroso/free-disk-space@54081f138730dfa15788a46383842cd2f914a1be
         with:
           # This might remove tools that are actually needed, if set to "true" but frees about 6 GB