nix.checks: create clean switch-configuration source

Adds `switchConfigurationSrc` source to checks that can be used for
other checks. This will replace derivations that use `self` for the
source which creates a new derivation every time anything in the
repository changes.

nix/checks/default.nix

@@ -28,6 +28,15 @@ let
       ../../switch-configuration
     ];
   };
+
+  # Used for derivations where switch-configuration is the primary directory.
+  switchConfigurationSrc = toSource {
+    root = ../..;
+    fileset = unions [
+      ../../facts
+      ../../switch-configuration
+    ];
+  };
 in
 genAttrs
   [