gitlab: fix potfile job for new location

Since we're storing the libvirt.pot file in git now, we should pull from
the source dir, not the build dir.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -210,7 +210,7 @@ potfile:
     - $MAKE -C src generated-sources
     - $MAKE -C po libvirt.pot
     - cd ..
-    - mv build/po/libvirt.pot libvirt.pot
+    - cp po/libvirt.pot libvirt.pot
   image: quay.io/libvirt/buildenv-libvirt-centos-8:latest
   artifacts:
     expose_as: 'Potfile'