Enforce concurrency, bash and use of color.

.github/workflows/ci.yml

@@ -1,7 +1,18 @@
 name: CI
-
 on: [pull_request]
 
+# Cancel active CI runs for a PR before starting another run
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: bash  # https://github.com/beeware/briefcase/pull/912
+
+env:
+  FORCE_COLOR: "1"
+
 jobs:
   pre-commit:
     runs-on: ubuntu-latest