tests: reduce unnecessary rebuilds of several tests

Some test were referring to the whole source code via ${self} which amde them rebuild on every single commit.

This is not mitigated by introduceing `self.filter { include = [...]; }` allowin to a content addressed subset of the source code in tests.

checks/flake-module.nix

@@ -1,6 +1,12 @@
-{ self, ... }:
+{ self, lib, ... }:
+let
+  inherit (lib)
+    filter
+    pathExists
+    ;
+in
 {
-  imports = [
+  imports = filter pathExists [
     ./backups/flake-module.nix
     ./devshell/flake-module.nix
     ./flash/flake-module.nix