8342988: GHA: Build JTReg in single step

[shipilev]

WIP. Makes sure GHA is more reliable. The original PR does not apply cleanly, because Alpine support added by JDK-8282944 is not in JDK 21.

Additional testing:

• GHA

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8342988 needs maintainer approval

Issue

• JDK-8342988: GHA: Build JTReg in single step (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/1269/head:pull/1269
$ git checkout pull/1269

Update a local copy of the PR:
$ git checkout pull/1269
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/1269/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 1269

View PR using the GUI difftool:
$ git pr show -t 1269

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk21u-dev/pull/1269.diff

[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.