RFR: 8263667: Avoid running GitHub actions on branches named pr/*

[Robin Westberg]

When the Skara feature "dependent pull requests" is activated for the JDK repository, branches with the name "pr/<number>" will start to appear. These will not be synced into personal forks by the Skara sync command, but if they are synced manually, we should avoid running GitHub actions workflows on them.

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Commit messages:
 - Filter out branches named pr/ for GitHub actions

Changes: https://git.openjdk.java.net/jdk/pull/3024/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3024&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263667
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3024.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3024/head:pull/3024

PR: https://git.openjdk.java.net/jdk/pull/3024