[jdk18u] RFR: 8282225: GHA: Allow one concurrent run per PR only

[Aleksey Shipilev]

Clean backport to improve GHA performance.

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Commit messages:
 - Backport aaab2cb41666af8520fd01de70e2c4f9c87ef5fd

Changes: https://git.openjdk.java.net/jdk18u/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18u&pr=37&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282225
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk18u/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18u pull/37/head:pull/37

PR: https://git.openjdk.java.net/jdk18u/pull/37