Add CI workflow

[eureka-cpu]

The flake checks can be the same as what's in the cloud-reaper workflow file. Other checks should run parts of nix-watch itself and look for specific things, e.g:

nix-watch -c
touch <some file>

should cause the current process to be killed, resulting in an error: interrupted by user if the process takes long enough. If the output of nix-watch is captured to a file, a grep of the file for that error could be one way of checking it's working. Another way to test that the -c, --clear flag works would be to check the status of the PID.

[eureka-cpu]

There's a bash testing framework called bat that could aid in testing functionality. This could be its own issue though so I'll link that here.

[eureka-cpu]

#19