writers: Allow string paths

Before this change, it was not possible to use string paths,
because then the `types.str.check` would succeed. So the only paths that
could be used were ones from the local filesystem via e.g.
`./some/path`.

We can easily fix this by using `types.path.check` instead to determine
whether we are dealing with a path.

This notably also allows using Nix-fetched sources as the content, e.g.
`fetchFromGitHub { ... } + "/some/file"`

pkgs/build-support/writers/default.nix

@@ -15,12 +15,12 @@ rec {
       name = last (builtins.split "/" nameOrPath);
     in
 
-    pkgs.runCommandLocal name (if (types.str.check content) then {
-      inherit content interpreter;
-      passAsFile = [ "content" ];
-    } else {
+    pkgs.runCommandLocal name (if types.path.check content then {
       inherit interpreter;
       contentPath = content;
+    } else {
+      inherit content interpreter;
+      passAsFile = [ "content" ];
     }) ''
       # On darwin a script cannot be used as an interpreter in a shebang but
       # there doesn't seem to be a limit to the size of shebang and multiple