Bump the rust-stable version to 1.64.0
This will probably not fix the coverage report (see #905 (closed)), but the 1.64 release is well in advance of the next GNOME release - so let's do it.
Fixes #904 (closed) by using just cargo test
for the non-full-build jobs. Adds a Fedora/x86_64 and an openSUSE/aarch64 job.