fix(ci): Fetch all refspecs from remote

This is a follow-up fix to ca298a2 and

Previously, 'git fetch' would not fetch all refspecs from the remote,
which would cause the 'Check if Dockerfile has been modified' step to
fail with the following error:

$ git diff --compact-summary --exit-code <good ref> -- Dockerfiles/<image>
fatal: bad object <good ref>

.github/workflows/linux.yml

@@ -42,7 +42,7 @@ jobs:
         run: |
           set -ex
           git remote add upstream https://github.com/nuttyartist/notes.git
-          git fetch upstream master
+          git fetch upstream +refs/heads/master:refs/remotes/upstream/master
           # NOTE: The following should give us the previous commit hash of the base branch, but that will
           #       only work reliably for 'push' and pull_request events. For workflow_dispatch events, we
           #       have to fallback to the default branch ('master'), until a better solution is found...