RFR: 8293338: Enable manually triggering the GitHub workflow

[Guoxiong Li]

Hi all,

In [8293214: Add support for Linux/LoongArch64](https://github.com/openjdk/jfx/pull/888), the contributor was suggested  to push a empty commit to trigger the GHA tests. But actually it seems not a good dev flow because the newly added commits are not related to the issue the patch want to solve.

The GitHub provides related configuration to enable the develoers to trigger the workflow manually. Please see the documents [1][2] for more detail. This patch adds such config to enable it.

Thanks for taking the time to review.

[1] https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow
[2] https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8293338

Changes: https://git.openjdk.org/jfx/pull/891/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=891&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293338
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/891.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/891/head:pull/891

PR: https://git.openjdk.org/jfx/pull/891