RFR: 8378702: jdk.test.lib.Platform.isMusl() may return false negative on Alpine Linux

[Frederic Thevenet]

jdk.test.lib.Platform.isMusl() may return false on Alpine linux, where it should return true.
This means that the jtreg tests is is used to guard will incorrectly fail.
This happens when tests deliberately clobber the  PATH and `ldd` can no longer be found. This PR uses a full path for `ldd` instead to work around the issue.

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Commit messages:
 - 8378702: jdk.test.lib.Platform.isMusl() may return false negative on Alpine Linux

Changes: https://git.openjdk.org/jdk/pull/29923/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29923&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378702
  Stats: 10 lines in 1 file changed: 2 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/29923.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29923/head:pull/29923

PR: https://git.openjdk.org/jdk/pull/29923