RFR: 8299330: Avoid installing MSYS2 in the JDK's source directory

[Julian Waters]

MSYS2 should be more appropriately installed into the runner's temporary directory rather than inside the newly checked out repository containing all the JDK's source code, as doing so may interfere with the build process and cause weird corner cases in the submitted actions workflow

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Commit messages:
 - Avoid installing MSYS2 in the JDK's source directory

Changes: https://git.openjdk.org/jdk/pull/11782/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11782&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299330
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11782.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11782/head:pull/11782

PR: https://git.openjdk.org/jdk/pull/11782