use coursier instead of sbt for Scala 3 scripts

[SethTisue]

thanks to scala/scala3#13081, and using the stuff I worked out at coursier/coursier#2058 (comment)

[SethTisue]

note that for now, the .scala file remains separate from the shell script that runs it. it would be nice if we only needed one file instead of two.

possible ways to get there:

• use some shell magic to have both parts in the same file and split the Scala part off at runtime for passing to the coursier invocation
• switch to Ammonite
• wait for the Standalone Scala effort to bear fruit

but meh, in the community build context this is already pretty satisfying. in other contexts the two-files thing would be more annoying, but here I don't really mind

[SethTisue]

small fixup to make this work on our Jenkins: cffdd5e