ci: Consider the HEAD commit hash for our cache test image tag

For the development branches of taskwarrior, caching 2.5.2 or 2.6.0 is
not sufficient since HEAD is still moving.

Build the tag hash based on the HEAD hash of the development branch in
question.

.github/workflows/tests.yaml

@@ -24,14 +24,20 @@ jobs:
       - uses: actions/checkout@v2
       - name: Prepare docker image
         run: |
-          set -ex -o pipefail
           shopt -s lastpipe
+          (
+            echo -n sha:
+            git ls-remote https://github.com/GothenburgBitFactory/taskwarrior.git | \
+            ( grep heads/$TASK_VERSION || : ) | awk '{print $1}'
+          ) | read -r TASK_DEVEL_SHA _ || :
+          echo "$TASK_DEVEL_SHA"
           (
             echo ALPINE_VERSION="$ALPINE_VERSION"
             echo PYTHON_VERSION="$PYTHON_VERSION"
             echo TASK_VERSION="$TASK_VERSION"
             echo VIM_VERSION="$VIM_VERSION"
             echo VIMWIKI_VERSION="$VIMWIKI_VERSION"
+            echo TASK_DEVEL_SHA="$TASK_DEVEL_SHA"
             cat Dockerfile
           ) | sha256sum | read -r tag _
           docker login "$DOCKER_REGISTRY" -u "$GITHUB_USER" -p "$GITHUB_TOKEN" || :