CI: run scala-native only on release

[kitbellew]

scala-native doesn't need to run on push since we don't need to upload artifacts, and building is already verified as part of test.

[tgodzik]

Should we also do that for native image?

[kitbellew]

Should we also do that for native image?

we could. here's what i thought:

1. we will remove it soon, so might as well not touch it :)
2. scala-native is always built as part of the test job (and then run), so the only difference is copying and then uploading which we don't need to test
3. native-image, however, involves some other steps which could potentially broken and only discovered after a release, so might be a reasonable check to keep for now.

let me know what you think.

[tgodzik]

Makes sense, let's keep it then