RFR: 756: Fix action platform names

[Robin Westberg]

The latest update to the GitHub Actions file accidentally swapped windows and mac.

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Commit messages:
 - Fix action platform names

Changes: https://git.openjdk.java.net/skara/pull/863/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=863&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-756
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/863.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/863/head:pull/863

PR: https://git.openjdk.java.net/skara/pull/863