.drone.yml: Give more resources to the JVM used by sbt
.drone.yml: Give more resources to the JVM used by sbt

sbt runs up to `Runtime.getRuntime.availableProcessors` tasks in
parallel, on our CI that means 40 tasks, compilation tasks are the one
with the biggest footprint and the introduction of
`dotty-library-bootstrapped` and `dotty-compiler-bootstrapped` in this
PR means that sbt can now run more compilation tasks in parallel. To
prevent the JVM from exploding, we increase:
- The max heap size, from 1G to 4G
- The max code cache size, from 240M to 512M
- The max metaspace size, from 256M to 1G

sjrd/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,som-snytt/dotty,lampepfl/dotty