Reduce duplication in .gitlab-ci.yml

3 jobs for mac in 2 minutes and 36 seconds (queued for 16 seconds)
Status Name Job ID Coverage
  Build
canceled build_mac: [bundled] #412663
mac

00:02:25

failed build_mac: [bundled gui] #412662
mac

00:00:11

 
  Test
canceled test_backend_mac #412664
mac

 
Name Stage Failure
failed
build_mac: [bundled gui] Build
$ cargo build --release --features $FEATURES
error: Found argument 'gui' which wasn't expected, or isn't valid in this context

USAGE:
cargo build --features <FEATURES>... --release

For more information try --help
Cleaning up file based variables
ERROR: Job failed: exit status 1