RFR: 8325444: GHA: JDK-8325194 causes a regression

[Christoph Langer]

The [change](https://github.com/openjdk/jdk/commit/d1c82156ba6ede4b798ac15f935289cfcc99d1a0) for [JDK-8325194](https://bugs.openjdk.org/browse/JDK-8325194) broke get-jtreg because the macro to determine the build jdk was not correct.

I fix this by reverting to the originally proposed way to use `$JAVA_HOME_17_<arch>`. I tried to use the bootstrap JDK in first place with the correct wiring but it turned out that the build of JTReg fails with that version, see [here](https://github.com/RealCLanger/jdk/actions/runs/7820130826/job/21334120478).

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Commit messages:
 - JDK-8325444

Changes: https://git.openjdk.org/jdk/pull/17756/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17756&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325444
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17756.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17756/head:pull/17756

PR: https://git.openjdk.org/jdk/pull/17756