Only run CI on latest commit of PR branches. (#913)

Same as RustCrypto/traits#1948.

Author: carloskiki

.github/workflows/crypto-bigint.yml

@@ -15,6 +15,11 @@ env:
   RUSTFLAGS: "-Dwarnings"
   RUSTDOCFLAGS: "-Dwarnings"
 
+# Cancels CI jobs when new commits are pushed to a PR branch
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: ubuntu-latest