Check Nix flake workflow (#46)

## Description of changes

<!-- What changes did you make -->

Add a workflow that runs on pr and when the flake.nix or lockfile change

## Relevant Issues

<!-- Eg. #43 -->

.github/workflows/check_flake.yml

@@ -0,0 +1,15 @@
+name: Check Nix flake
+on:
+ pull_request:
+ push:
+ paths:
+ - 'flake.lock'
+ - 'flake.nix'
+
+jobs:
+ check:
+ runs-on: ubuntu-latest
+ steps:
+ - uses: actions/checkout@v3
+ - uses: cachix/install-nix-action@v20
+ - run: nix flake check