Usage tip for mono: Non-build jobs (like uploading to itch) should use non-mono image

[realkotob]

The mono image is significantly larger. Using the non-mono image for jobs other than building results in almost a minute less time for each of these jobs.

It is possible to change the image for separate jobs manually but it has to be added to each job separately as an extra specifier.

We currently only have one gitlab-ci.yml script in the repo so it would be weird to change it since it would make the non-mono users confused. It would work better as a tip in the README file.

I wonder if we can use an ENV variable as an image reference and have that be used for non-build jobs? That would make it so there's only one extra line that needs to be changed when updating the version, and wouldn't annoy non-mono users as much.