[jdk21u] RFR: 8313707: GHA: Bootstrap sysroots with --variant=minbase

[Aleksey Shipilev]

Clean backport to improve and fix GHA.

Additional testing:
 - [ ] GHA

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Commit messages:
 - Backport 29f1d8ef50e3b4b235177fd9642a1fcf26fe3bab

Changes: https://git.openjdk.org/jdk21u/pull/43/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=43&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313707
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/43.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/43/head:pull/43

PR: https://git.openjdk.org/jdk21u/pull/43