RFR: Update GHA for panama branch

[Jorn Vernee]

Port of https://github.com/openjdk/jextract/pull/134 + some polish such as using the latest jtreg version, using the latest version of the setup-java and checkout actions, and avoiding use of explicit version numbers in variable names, to avoid having to update all of them again in the future.

Historically we've had the github tests disabled on the panama branch, since we technically require a JDK from that panama-foreign repo.

However, right now there is no difference between the Panama repo and JDK22, so we can use the JDK22 EA binary temporarily, and get some extra GHA testing.

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Commit messages:
 - Missed renaming of one variable
 - Polish
 - Update GHA for jdk22

Changes: https://git.openjdk.org/jextract/pull/147/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=147&range=00
  Stats: 20 lines in 1 file changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/jextract/pull/147.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/147/head:pull/147

PR: https://git.openjdk.org/jextract/pull/147