ci(lint): exclude .in files from ShellCheck lint

Exclude all `.in` files because they may contain unsupported syntax, and
they have to be preprocessed first. For example:

```sh
Error: SHELLCHECK_WARNING:
./src/rpm/systemd-update-helper.in:130:37: warning[SC1083]: This { is literal. Check expression (missing ;/\n?) or quote it.
```

Related to: systemd/systemd#28521

(cherry picked from commit 97eb826)

Related: RHEL-1086

.github/workflows/differential-shellcheck.yml

@@ -32,4 +32,6 @@ jobs:
       - name: Differential ShellCheck
         uses: redhat-plumbers-in-action/differential-shellcheck@v4
         with:
+          # exclude all `.in` files because they may contain unsupported syntax, and they have to be preprocessed first
+          exclude-path: '**/*.in'
           token: ${{ secrets.GITHUB_TOKEN }}