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

[Aleksey Shipilev]

On Thu, 10 Mar 2022 08:26:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Clean backport to improve GHA performance.

Got the push approval.

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