sbt-dotty: Fix the binary suffix used for publishing

[smarter]

The compiler we publish is compiled with a non-bootstrapped compiler, so
its scalaBinaryVersion suffix is computed from the version number of the
non-bootstrapped compiler, for 3.0.0-RC3 this was
3.0.0-RC3-nonboostrapped and we correctly return 3.0.0-RC3 as a
binary version, but for 3.0.0 this is 3.0.0-nonbootstrapped for which
we return again 3.0.0-nonbootstrapped when we really want just 3.
This is why we incorrectly published https://repo1.maven.org/maven2/org/scala-lang/scala3-compiler_3.0.0-nonbootstrapped/

Fixed by special-casing RCs and milestones which are the only special
suffixes that get their own binary suffix.

This fix is specific to the release-3.0.0 branch since the master branch
does not use sbt-dotty anymore (and will need its own fix to publish
3.0.1-RC1 correctly due to a different issue with how sbt computes
the suffix for 3.0.1-RC1-nonbootstrapped).

[smarter]

mill fails because it's also incorrectly setting the scalaBinaryVersion for 3.0.0-bin-..., which we can ignore since it shouldn't happen for the release. However I don't understand the scodec failure, I see it's been worked around in a9f1fa6 to not affect the release build so we can also ignore it, but I don't understand why it worked previously since it's using an experimental feature without the corresponding language.experimental flag being enabled., oh well.

[smarter]

mill fails because it's also incorrectly setting the scalaBinaryVersion for 3.0.0-bin-..., which we can ignore since it shouldn't happen for the release.

I was wrong, that also fails in release builds because the community build is still done with 3.0.0-bin-SNAPSHOT, it didn't fail before because both our own build and mill were using the same incorrect suffix.