Only run CI on latest commit of PR branches (#553)

Same as RustCrypto/traits#1948.

Author: carloskiki

.github/workflows/ci.yml

@@ -10,6 +10,11 @@ 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

.github/workflows/workspace.yml

@@ -9,6 +9,11 @@ on:
     paths-ignore:
       - README.md
 
+# 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:
   clippy:
     runs-on: ubuntu-latest