ci: move vars to the jobs
ci: move vars to the jobs

The gitlab parser will only expand the variables once,
which make it impossible to abstract over the rust
version in a nice way. A hacky workaround is to
duplicate some things and get rid of the RUST_VERSION
var to avoid the issue.

https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/618#note_679774

sdroege/rsplugin,sdroege/rsplugin