RFR: 8313707: GHA: Bootstrap sysroots with --variant=minbase

[Aleksey Shipilev]

We don't need the fully bootable Debian installation to build against. Therefore, we can bootstrap with `--variant=minbase`. The sysroots are about 5..10% smaller then.

This would also make our builds more immune to issues like [JDK-8313701](https://bugs.openjdk.org/browse/JDK-8313701). Note that RISC-V GHAs are fixed with this PR.

Additional testing:
 - [x] GHA cross-compilation builds

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/15140/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15140&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/jdk/pull/15140.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15140/head:pull/15140

PR: https://git.openjdk.org/jdk/pull/15140