RFR: CODETOOLS-7903112: jcstress: GHA: Allow one concurrent run per PR only

[Aleksey Shipilev]

Similar to [JDK-8282225](https://bugs.openjdk.java.net/browse/JDK-8282225), we want the same for jcstress.

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jcstress/pull/113/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=113&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903112
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/113.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/113/head:pull/113

PR: https://git.openjdk.java.net/jcstress/pull/113