Cache sbt files in CI, update versions of CI actions

[jiribenes]

Investigating #428 since I noticed that setup-java allows simple sbt caching when playing with https://github.com/effekt-lang/effekt/pull/538 :)

[jiribenes]

It's very difficult to measure if this helps at all, I'm guesstimating based on only 5 runs. On a cache hit, this PR makes the CI go from around ~11m40-12m55 to ~11m20-12m15. On a cache miss, it takes much longer (15m00), but we shouldn't have a cache miss too often (our sbt dependencies, Java/Scala versions are stable)

Regarding what could be done next:

• Caching MLton makes very little sense (it takes only 1s already)
• similarly for apt dependencies (takes about 10s; what we could do there is to put them into a single command...)

[jiribenes]

I think it's worth merging this, even if the speed-up isn't too huge, but I'll leave #428 open as yet unresolved. I'll also back-port these changes into #538 :)