Make the CI faster

[jvican]

• Speed up git cloning: Mitigate #2387: Improve cloning logic scala/scala3#3850
• Split up our gigantic test suite into chunks we can run in parallel (probably two jobs is enough)
• Share the bridge by setting the sbt home global in the scripted tests

[jvican]

Would be great to share the bridge and what not, but CI is faster enough not to bother with this.