fetchzip: fix extraPostFetch concatenation

4a5c49363a added some more commands after
`extraPostFetch` but concatenated them without a separating newline.

Which means, that since that commit

  fetchzip { ..., extraPostFetch = ''rm -f "$out"/some-file''; }

now actually runs the following shell command

  rm -f "$out"/some-file"chmod -R a-w "$out"

thus deleting "$out". Which is very unfortunate.

Especially since this actually happens on master for all `fetchFromBitbucket`
derivations. But since the results are fixed-output users bulding with hydra
cache enabled are not hitting this for not recently updated derivations yet.

pkgs/build-support/fetchzip/default.nix

@@ -45,7 +45,9 @@
     '' else ''
       mv "$unpackDir" "$out"
     '')
-    + extraPostFetch
+    + ''
+      ${extraPostFetch}
+
       # Remove write permissions for files unpacked with write bits set
       # Fixes https://github.com/NixOS/nixpkgs/issues/38649
       #
@@ -54,7 +56,6 @@
       # single-user Nix installations. This is because in sandbox mode we'll try to
       # move the path, and if we don't have write permissions on the directory,
       # then we can't update the ".." entry.
-    + ''
       chmod -R a-w "$out"
       chmod u+w "$out"
     '';