Allow setting of custom arguments

[shinyhappydan]

I am using the action on a self-hosted runner which requires proxy settings. I don't seem to be able to set these, meaning coursier fails when installing apps

I tried to set JAVA_OPTS, however this doesn't seem to be possible (not sure if this would work anyway)

My understanding is, I would need to get coursier to run like this: coursier -J-Dhttps.proxyHost=… -J-Dhttps.proxyPort=… as documented here

If this makes sense as a useful feature, I'd be willing to work on a pull request. Let me know what you think

[alexarchambault]

@shinyhappydan Sure, that could be helpful