ci: simplify concurrency string

`github.ref` is now defined for both pull requests and pushes, meaning
that it can be used to simplify the concurrency group.
`cancel-in-progress` is set to true only if the trigger is a pull
request, as we don't want master runs to cancel each other out.

.github/workflows/optional.yml

@@ -5,8 +5,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
-  cancel-in-progress: true
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 env:
   INSTALL_PREFIX: ${{ github.workspace }}/nvim-install