feat(ci): enable concurrency for nix-tests

Don't let multiple tests run, cancel existing for same pr

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

.github/workflows/nix-tests.yml

@@ -2,6 +2,9 @@ name: "Nix-Tests"
 on:
   pull_request:
   push:
+concurrency:
+  group: ${{ github.workflow }}-nix-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 jobs:
   nix-flake-check:
     runs-on: ubuntu-latest