The registry path is lower case, so it can be different from `FDO_UPSTREAM_REPO`

Found on the GNOME gitlab:

For projects like mutter, FDO_UPSTREAM_REPO is set to GNOME/mutter. However, the registry url is for example registry.gitlab.gnome.org/gnome/mutter/master.

The templates should lower case the FDO_UPSTREAM_REPO upstream repo so skopeo doesn't get lost.

We still need to keep FDO_UPSTREAM_REPO untouched, because we embed this as a label in the created image, so changing it there might have undesirable side effects.

Cc: @jadahl