build: improve the format-cpp error message

This change specifies the target branch name instead of leaving that to
the committer to fill in.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #42765
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

.github/workflows/linters.yml

@@ -78,7 +78,7 @@ jobs:
             echo
             echo 'ERROR: Please run:'
             echo
-            echo '  CLANG_FORMAT_START="$(git merge-base HEAD <target-branch-name>)" make format-cpp'
+            echo "  CLANG_FORMAT_START="$\(git merge-base HEAD ${GITHUB_BASE_REF}\)" make format-cpp"
             echo
             echo 'to format the commits in your branch.'
             exit "$EXIT_CODE"