ci: fix CI skip script hole (#21741)

In some environments, the script will not fail despite SKIP_CHECK_BRANCH
being unset, leading to the script explicitly skipping CI when it should
fail fast.

Prevent this by explicitly checking for the env var.

.github/scripts/check_skip_ci.sh

@@ -13,7 +13,8 @@ set -euo pipefail
 #
 # ... `git merge-base origin/$SKIP_CHECK_BRANCH HEAD` would return commit `D`
 # `...HEAD` specifies from the common ancestor to the latest commit on the current branch (HEAD)..
-files_to_check=$(git diff --name-only "$(git merge-base origin/$SKIP_CHECK_BRANCH HEAD~)"...HEAD)
+skip_check_branch=${SKIP_CHECK_BRANCH:?SKIP_CHECK_BRANCH is required}
+files_to_check=$(git diff --name-only "$(git merge-base origin/$skip_check_branch HEAD~)"...HEAD)
 
 # Define the directories to check
 skipped_directories=("docs/" "ui/" "website/" "grafana/" ".changelog/")