GitHub: cancel jobs for previous pushes of PR

This takes over two more settings we have in the lnd repo. The first is
to cancel existing CI runs for the same PR if it is pushed again before
the previous run has completed.

The second is just for consistency, to make sure all shells are bash
shells.

.github/workflows/main.yml

@@ -8,6 +8,15 @@ on:
     branches:
       - "*"
 
+concurrency:
+  # Cancel any previous workflows if they are from a PR or push.
+  group: ${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: bash
+
 env:
   # If you change this value, please change it in the following files as well:
   # /Dockerfile