RFR: 8315062: [GHA] get-bootjdk action should return the abolute path

[Xin Liu]

We are running hotspot:tier2 on github action(GHA). I have difficulty to execute this test on GHA:
Runtime/cds/appcds/dynamicArchive/TestAutoCreateSharedArchiveUpgrade.java 

In GHA, the java property is gained from environment variable BOOT_JDK.
BOOT_JDK=${{ steps.bootjdk.outputs.path }}

it is in turn from action get-bootjdk. now it returns a relative path. The aforementioned test has trouble to find the executable if the path relative.
I would recommend changing the action get-bootjdk. we just return the absolute path.

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Commit messages:
 - 8315062: [GHA] get-bootjdk action should return the abolute path

Changes: https://git.openjdk.org/jdk/pull/15435/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315062
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15435/head:pull/15435

PR: https://git.openjdk.org/jdk/pull/15435