Trigger CI on push and PR

Problem: CI is triggered only on push, so it won't work in case of
PRs from forks.
Solution: trigger it on PRs as well. The drawback is that it may
run twice, but this repo is not actively worked on and doesn't have
much code, so it sounds fine.

.github/workflows/check.yml

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: MPL-2.0
 name: Nix flake check
-on: push
+on: [push, pull_request]
 
 jobs:
   validate: