RFR: CODETOOLS-7903233: jcstress: Improve GHA triggers

[Aleksey Shipilev]

GHA workflows need to have a push-based trigger instead of pull_request one, in order to comply with current scheme of running actions in private forks.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/116/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=116&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903233
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jcstress/pull/116.diff
  Fetch: git fetch https://git.openjdk.org/jcstress pull/116/head:pull/116

PR: https://git.openjdk.org/jcstress/pull/116