bug: GetAntlrJar fails to locate jar file on (some?) *nix systems

[mrfootoyou]

The GetAntlrJar task fails to locate the Antlr .jar file when compiling on *nix systems, specifically Ubuntu and MacOS.

• Antlr4BuildTasks 8.9
• Ubuntu 18.04, 20.04, MacOS 10.15 (Github workers)
• .NET 3.1, 5.0

I've created a repo which demonstrates the error via Github actions. This run shows the build failure and its proximate cause: System.IO.File.Exists(uri.LocalPath) returns false. You can also view the detailed build output here. Also note that the attempts to download the package directly from maven all failed. I'm not sure if that is expected or not. Perhaps the same error is preventing the downloaded files from being detected??

Unfortunately I don't know enough about *nix systems to offer a specific solution to the problem, but I would recommend that you allow the user to specify the UsingToolPath via an optional MSBuild parameter -- had this existed I could have manually computed the correct path as a workaround.

On a related topic, it might be a good idea to use the JAVA_HOME environment variable, if defined, for UsingJavaExec instead of hardcoding it to /usr/bin/java. Also allow the user to override the path via an optional MSBuild parameter just in case.