GitHub Actions caching causing slowdown and spurious failures in CI

[griggt]

The CI job steps using GitHub Actions actions/cache@v1.1.2 are intermittently running very slowly, and occasionally causing spurious failures due to failed cache restoration.

This has been occurring for many weeks now.

A recent example can be found here: https://github.com/lampepfl/dotty/runs/1375585886?check_suite_focus=true

In this case, 45 minutes were spent attempting to restore the cache, and to top if off, an error occurred during the Cache SBT action (after 31 minutes!):

gzip: stdin: unexpected end of file
/usr/bin/tar: Unexpected EOF in archive
/usr/bin/tar: Unexpected EOF in archive
/usr/bin/tar: Error is not recoverable: exiting now
Warning: Tar failed with error: The process '/usr/bin/tar' failed with exit code 2

which I believe is the cause of this subsequent failure in the Test step:

Exception in thread "sbt-parser-init-thread" java.lang.ExceptionInInitializerError
	at sbt.internal.parser.SbtParserInit$$anon$2.run(SbtParser.scala:171)
Caused by: scala.reflect.internal.FatalError: Error accessing /root/.sbt/0.13/java9-rt-ext-private_build_14_0_1/rt.jar
  ...
Caused by: java.util.zip.ZipException: zip END header not found
  ...
Error:  java.lang.NoClassDefFoundError: Could not initialize class sbt.internal.parser.SbtParser$
Error:  Use 'last' for the full log.
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