Reenable our own cache

.github/actions/setup-go/action.yml

@@ -7,15 +7,47 @@ runs:
       uses: actions/setup-go@v5
       with:
         go-version: '1.22'
+        # unfortunately we cannot use the provided caching because it uses the
+        # same cache for all workflows/jobs, leading to undesired cache restores
+        # causing uncached test runs etc ...
+        # You can see the cache key at https://github.com/actions/setup-go/blob/4e0b6c77c6448caafaff5eed51516cad78e7639a/src/cache-restore.ts#L34
+        cache: false
 
     # Restore original modification time of files based on the date of the most
     # recent commit that modified them as mtimes affect the Go test cache.
     # See https://github.com/golang/go/issues/58571 for details
     - name: Restore modification time of checkout files
       uses: chetan/git-restore-mtime-action@075f9bc9d159805603419d50f794bd9f33252ebe
 
+
+    # KEY_PREFIX must uniquely identify the specific instance of a job executing.
+    - shell: bash
+      run: |
+        echo 'KEY_PREFIX=${{ github.workflow }}-${{ github.job }}-${{ runner.os }}-${{ inputs.go-version }}-matrix(${{ join(matrix.*,'|') }})' >> $GITHUB_OUTPUT
+        echo "GOCACHE=$(go env GOCACHE)" >> $GITHUB_OUTPUT
+        echo "GOMODCACHE=$(go env GOMODCACHE)" >> $GITHUB_OUTPUT
+      id: variables
+
+    # Cache the Go Modules downloaded during the job.
+    - uses: actions/cache@v4
+      with:
+        path: ${{ steps.variables.outputs.GOMODCACHE }}
+        key: ${{ steps.variables.outputs.KEY_PREFIX }}-go-mod-${{ hashFiles('**/go.sum') }}
+        restore-keys: ${{ steps.variables.outputs.KEY_PREFIX  }}-go-mod-
+
+    # Cache any build and test artifacts during the job, which will speed up
+    # rebuilds and cause test runs to skip tests that have no reason to rerun.
+    - uses: actions/cache@v4
+      with:
+        path: ${{ steps.variables.outputs.GOCACHE }}
+        key: ${{ steps.variables.outputs.KEY_PREFIX  }}-go-build-${{ github.ref }}-${{ hashFiles('**', '!.git') }}
+        restore-keys: |
+          ${{ steps.variables.outputs.KEY_PREFIX  }}-go-build-${{ github.ref }}-
+          ${{ steps.variables.outputs.KEY_PREFIX  }}-go-build-
+
     # Reset the cache for master/protected branches, to ensure they build and run the tests from zero
     # and that the module cache is cleaned (otherwise it accumulates orphan dependencies over time).
     - if: github.ref_protected
       shell: bash
-      run: sudo rm -rf ~/.cache/go-build ~/go/pkg/mod
+      run: sudo rm -rf ${{ steps.variables.outputs.GOMODCACHE }} ${{ steps.variables.outputs.KEY_PREFIX  }}
+