[FLINK-13374][scala][build] Set -Xss2m when compiling scala

[zentol]

This PR bumps the ThreadStackSize to 2MB when compiling scala. This appears to be the common approach for handling StackOverflowErrors by the scala compiler.

see https://www.scala-lang.org/old/node/10931, scala/bug#10604

I haven't observed an increase in compile time.

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CI report:

• cc907a6 : FAILURE Build