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.

lib/inventory/flake-module.nix

@@ -46,7 +46,19 @@ in
           nix-unit --eval-store "$HOME" \
             --extra-experimental-features flakes \
             ${inputOverrides} \
-            --flake ${self}#legacyPackages.${system}.evalTests-inventory
+            --flake ${
+              self.filter {
+                include = [
+                  "flakeModules"
+                  "lib/default.nix"
+                  "lib/flake-module.nix"
+                  "lib/inventory"
+                  "lib/frontmatter"
+                  "clanModules/flake-module.nix"
+                  "clanModules/borgbackup"
+                ];
+              }
+            }#legacyPackages.${system}.evalTests-inventory
 
           touch $out
         '';