RFR: 8310183: Update GitHub Actions to use boot JDK for building jtreg

[Jonathan Gibbons]

On Fri, 16 Jun 2023 06:05:16 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Please review this change to use the boot JDK for building jtreg when running on GitHub Actions.
> 
> This is a best-effort follow-up change to
> - #14448
> which didn't have the desired results - the `Bad address` error does still appear with using the pre-installed JDKs 11 and 17.
> 
> Tests using the boot JDK for building jtreg seem to be successful and more stable.

To those investigating the msys issue, do we know why and where `CLASSPATH` is being used?   In general, that's a deprecated style of use.

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PR Comment: https://git.openjdk.org/jdk/pull/14507#issuecomment-1594821953