Fix the CI to use a PR version of sass-spec (#1201)

The previous implementation contained 2 mistakes:

- there was a mismatch between names of env variables in the workflow
  config and in the script
- the `github.base_ref` property in github actions contains a branch
  name, not a full git ref like `github.ref`

.github/workflows/ci.yml

@@ -42,7 +42,7 @@ jobs:
           git -C sass-spec checkout FETCH_HEAD
         env:
           PR_BRANCH: "${{ github.base_ref }}"
-          CURRENT_BRANCH: "${{ github.ref }}"
+          CURRENT_REF: "${{ github.ref }}"
           PR_BODY: "${{ github.event.pull_request.body }}"
       - uses: ruby/setup-ruby@v1
         with:

tool/github-action/sass-spec-ref.sh

@@ -8,15 +8,21 @@
 # sass-spec pull request, we'll run against the latter rather than sass-spec
 # master.
 
-GITHUB_REF=${PR_REF:-$CURRENT_REF}
-if [[ "$GITHUB_REF" == refs/heads/feature.* ]]; then
-  default="${GITHUB_REF:11}"
+if [ -z "$PR_BRANCH" ]; then
+  # Remove the "refs/heads/" prefix
+  current_branch="${CURRENT_REF:11}"
+else
+  current_branch="$PR_BRANCH"
+fi
+
+if [[ "$current_branch" == feature.* ]]; then
+  default="$current_branch"
 else
   default=master
 fi
 
-# We don't have a PR_REF so we are not in a pull request
-if [ "$GITHUB_REF" == "$CURRENT_REF" ]; then
+# We don't have a PR_BRANCH so we are not in a pull request
+if [ -z "$PR_BRANCH" ]; then
   >&2 echo "Ref: $default."
   echo "$default"
   exit 0