Unrevert some other changes

Author: Shay Bergmann

nix/pkgs/mantis/default.nix

@@ -12,9 +12,10 @@
           filter = path: _: !lib.hasSuffix "nix" path;
         };
     in
-    if isGit then
+    if lib.isStorePath src then src
+    else if isGit then
       if dirty then filterSrc (gitignoreSource src) else repo
-    else src;
+    else builtins.path { name = "source"; path = src; };
 
   mantis-source = mkSrc src;