RFR: 8254054: Pre-submit testing using GitHub Actions should not use the deprecated set-env command

[Robin Westberg]

The `set-env` command was recently deprecated in favor of a different method of setting environment variables, due to a
security vulnerability [1]. The vulnerability does not affect our usage of GitHub Actions, but we should nevertheless
update to avoid the associated deprecation warnings.

[1] https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

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Commit messages:
 - Remove usage of deprecated ::set-env

Changes: https://git.openjdk.java.net/jdk/pull/518/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=518&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254054
  Stats: 53 lines in 1 file changed: 1 ins; 21 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/518.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/518/head:pull/518

PR: https://git.openjdk.java.net/jdk/pull/518