RFR: 8292549: GitHub actions: intermittent build failure on macOS while downloading ant

[Kevin Rushforth]

Use the system version of `ant` on macOS platforms rather than downloading it from apache.org.

As noted in the bug report, we are seeing frequent GitHub Actions (GHA) failures on macOS due to a timeout while attempting to download apache ant 1.10.5.

The JavaFX build uses `ant` to build the apps (so `gradle apps` requires it). The installed version of ant on the macOS 11 systems that we use is ant 1.10.12, which works fine to build the apps. Given that the GHA builds are just a sanity test used as a help to reviewers and contributors, and that we already relax the requirement to specify the exact versions for a few other platform tools, there is no real downside to doing this.

This PR only touches the GHA build script for the macOS job, so the GHA test run is sufficient to test it.

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Commit messages:
 - 8292549: GitHub actions: intermittent build failure on macOS while downloading ant

Changes: https://git.openjdk.org/jfx/pull/879/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292549
  Stats: 16 lines in 1 file changed: 5 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/879.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/879/head:pull/879

PR: https://git.openjdk.org/jfx/pull/879