[CI] Cancel in progress jobs (plus readonly permissions) (#65)

Co-authored-by: Domenic Quirl <DomenicQuirl@protonmail.com>

.github/workflows/main.yml

@@ -10,6 +10,15 @@ on:
 
   # Allows to run this workflow manually from the Actions tab
   workflow_dispatch:
+
+permissions:
+  contents: read
+
+# If new code is pushed to a PR branch, then cancel in progress workflows for that PR. Ensures that
+# we don't waste CI time, and returns results quicker https://github.com/jonhoo/rust-ci-conf/pull/5
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
 
 env:
   RUST_LOG: info