ci: Simplify getting branch HEAD commit hash

This is simpler and more reliable/predictable:

"git ls-remote 'heads/'" doesn't return anything, while "grep 'heads/'"
returns all branches, so a taskwarrior push causes most docker images
(those that don't have any TASK_VERSION and fallback to the Dockerfile
default) to be rebuilt, not just the one that needs to be.

.github/workflows/tests.yaml

@@ -26,11 +26,7 @@ jobs:
         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 $1}'
-          ) | read -r TASK_DEVEL_SHA _ || :
+          git ls-remote https://github.com/GothenburgBitFactory/taskwarrior.git heads/"$TASK_VERSION" | read -r TASK_DEVEL_SHA _ || :
           (
             echo ALPINE_VERSION="$ALPINE_VERSION"
             echo PYTHON_VERSION="$PYTHON_VERSION"