RFR: 8343190: GHA: Try building JTReg several times

[Aleksey Shipilev]

We still sometimes have problems checking out JTReg build dependencies. [JDK-8342988](https://bugs.openjdk.org/browse/JDK-8342988) makes it less pronounced, but it still happens. We might need to do a retries for JTReg builds to make them more reliable.

Additional testing:
 - [x] GHA (seeing retries with deliberately broken JTReg build)
 - [x] GHA (passing JTReg build)

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Commit messages:
 - Work

Changes: https://git.openjdk.org/jdk/pull/21758/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21758&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343190
  Stats: 18 lines in 1 file changed: 16 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21758.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21758/head:pull/21758

PR: https://git.openjdk.org/jdk/pull/21758