add cancel in progress for CI

.github/workflows/cachix.yml

@@ -7,6 +7,10 @@ on:
     branches: ["main"]
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   nix:
     strategy:

.github/workflows/rust.yml

@@ -9,6 +9,10 @@ on:
 env:
   CARGO_TERM_COLOR: always
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   fmt:
     name: Formatting