RFR: CODETOOLS-7903234: JMH: 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.

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/75/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903234
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jmh/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jmh pull/75/head:pull/75

PR: https://git.openjdk.org/jmh/pull/75