RFR: 8256983: GitHub actions: specify the version of each platform OS and compiler

[Kevin Rushforth]

As described in the [JBS issue](https://bugs.openjdk.java.net/browse/JDK-8256983), we should specify the specific version of each OS and compiler rather than just using the defaults. This will help insulate us from changes to the defaults that can break the build, and has recently done so.

On Linux, we upgraded to 20.04 (18.04 is still the default), which required specifying the version of ant, since the default version for 20.04 is 1.10.7 which has a bug that causes FX apps to fail. I decided to also specify the version of ant (1.10.5) for all three platforms.

The following will be used:

| Platform | OS | Compiler | Ant |
| --- | --- | --- | --- |
| Linux | Ubuntu 20.04 | gcc 10.2 | ant 1.10.5 |
| Mac | macOS 10.15 | Xcode 11.3.1 | ant 1.10.5 |
| Windows | Windows Server 2019 | [1] | ant 1.10.5 |

[1] The Microsoft compiler version is unspecified as there is a dependency on [JDK-8255713](https://bugs.openjdk.java.net/browse/JDK-8255713) in addition to the problem of increasing the build time to download a specific version.

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Commit messages:
 - 8256983: GitHub actions: specify the version of each platform OS and compiler

Changes: https://git.openjdk.java.net/jfx/pull/366/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=366&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256983
  Stats: 46 lines in 1 file changed: 34 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jfx/pull/366.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/366/head:pull/366

PR: https://git.openjdk.java.net/jfx/pull/366