[chore] Upgrade Scala Native to 0.5.12

[WojciechMazur]

Upgrades default Scala Native verison to 0.5.12 (https://github.com/scala-native/scala-native/releases/tag/v0.5.12)

[Gedochao]

Downloading compiler plugin org.scala-native:::nscplugin:0.5.12
[error]  Error downloading org.scala-native:nscplugin_3.8.4-RC3:0.5.125.12/nscplugin_3.8.4-RC3-0.…
  not found: /Users/pchabelski/Library/Caches/ScalaCli/local-repo/1.14.0/org.scala-native/nscplugin_3.8.4-RC3/0.5.12/ivys/ivy.xml
  No fallback URL found
  not found: /Users/pchabelski/.ivy2/local/org.scala-native/nscplugin_3.8.4-RC3/0.5.12/ivys/ivy.xml
  not found: https://repo1.maven.org/maven2/org/scala-native/nscplugin_3.8.4-RC3/0.5.12/nscplugin_3.8.4-RC3-0.5.12.pom

nscplugin seems to not have been published for 3.8.4-RC2 or 3.8.4-RC3

[Gedochao]

Depends on:

• Bump Scala 3 Next RC to 3.8.4-RC3 #4288

(as the nscplugin isn't published for 3.8.4-RC2)