pull-local: Write detached metadata into the correct repository

We want to write to the dest repo, not src.

Noticed while reviewing this code for some other bug.

https://bugzilla.gnome.org/show_bug.cgi?id=733579

tests/test-basic.sh

@@ -326,6 +326,8 @@ rm repo2 -rf
 mkdir repo2
 ${CMD_PREFIX} ostree --repo=repo2 init
 ${CMD_PREFIX} ostree --repo=repo2 pull-local repo
+ostree --repo=repo2 show --print-detached-metadata-key=SIGNATURE test2 > test2-meta
+assert_file_has_content test2-meta "HANCOCK"
 echo "ok pull-local after commit metadata"
 
 cd ${test_tmpdir}