Reduce duplication in .gitlab-ci.yml

6 jobs for mac in 7 minutes and 17 seconds (queued for 50 minutes and 12 seconds)
latest
Status Name Job ID Coverage
  Build
failed build_mac: [bundled] #435205
mac

00:07:17

canceled build_mac: [bundled gui] #435173
mac

failed build_mac: [bundled] #435174
mac

00:03:21

failed build_mac: [bundled] #435176
mac

00:05:04

failed build_mac: [bundled] #435182
mac

00:07:05

 
  Test
skipped test_backend_mac #435175
mac
 
Name Stage Failure
failed
build_mac: [bundled] Build
    | 

Compiling tokio-tungstenite v0.13.0
Some errors have detailed explanations: E0308, E0658.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `tract-data` due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
error: build failed
Cleaning up file based variables
ERROR: Job failed: Process exited with status 1