RFR: 8310259: Pin msys2/setup-msys2 github action to a specific commit

[Jaikiran Pai]

Can I please get a review of this change which proposes to address the recent github actions failures?

As noted in https://bugs.openjdk.org/browse/JDK-8310259 the commit in this PR pins the `msys2/setup-msys2` to a previous release (that works) and also reverts the change that was done in https://github.com/openjdk/jdk/pull/14507, for reasons discussed in that PR.

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Commit messages:
 - add comment
 - use specific (working) commit of msys2 github action
 - Revert "8310183: Update GitHub Actions to use boot JDK for building jtreg"

Changes: https://git.openjdk.org/jdk/pull/14516/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310259
  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14516.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14516/head:pull/14516

PR: https://git.openjdk.org/jdk/pull/14516