pre-commit: properly exclude generated nix files

Author: vlaci

.pre-commit-config.yaml

@@ -1,6 +1,6 @@
 # See https://pre-commit.com for more information
 # See https://pre-commit.com/hooks.html for more hooks
-exclude: ^tests/integration|\.patch|nix/_sources$
+exclude: ^tests/integration|\.patch|^nix/_sources
 repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v4.4.0