Simplify build instructions: apply Maven heap settings automatically

[Sanne]

Maven can pick up the project defaults from .mvn/jvm.config : there is no need to ask everyone to set certain MAVEN_OPTS.

[gsmet]

Hmmm, this PR was merged with all the JVM builds failing?

https://github.com/quarkusio/quarkus/pull/12159/checks

[gsmet]

I reverted it.

[Sanne]

I checked all those logs and didn't find anything related.

[gsmet]

Well... when ALL the JVM tests fail, you can be sure you have an issue.

Just checking the first one gives me:

2020-09-17T10:51:04.5758436Z Expecting:
2020-09-17T10:51:04.5759054Z  <"Error occurred during initialization of VM
2020-09-17T10:51:04.5759831Z Initial heap size set to a larger value than the maximum heap size
2020-09-17T10:51:04.5760505Z ">
2020-09-17T10:51:04.5760778Z to contain:
2020-09-17T10:51:04.5761063Z  <"BUILD SUCCESS">
2020-09-17T10:51:04.5761446Z  (ignoring case)
2020-09-17T10:51:04.5763934Z 	at io.quarkus.scala.maven.it.ScalaRemoteDevModeIT.testThatTheApplicationIsReloadedOnScalaChange(ScalaRemoteDevModeIT.java:25)

which definitely looks related to the change, that's why I reverted.

And the Maven tests got berserk too:

2020-09-17T09:07:20.2556181Z Error occurred during initialization of VM
2020-09-17T09:07:20.2557328Z Initial heap size set to a larger value than the maximum heap size
2020-09-17T09:07:50.4385426Z Error occurred during initialization of VM
2020-09-17T09:07:50.4386549Z Initial heap size set to a larger value than the maximum heap size
2020-09-17T09:27:50.6451033Z Error occurred during initialization of VM
2020-09-17T09:27:50.6451903Z Initial heap size set to a larger value than the maximum heap size
2020-09-17T09:32:50.8795669Z Error occurred during initialization of VM
2020-09-17T09:32:50.8796722Z Initial heap size set to a larger value than the maximum heap size
2020-09-17T09:33:51.3295236Z Error occurred during initialization of VM
2020-09-17T09:33:51.3296196Z Initial heap size set to a larger value than the maximum heap size

[Sanne]

ahh, found it, sorry:

Initial heap size set to a larger value than the maximum heap size

[Sanne]

I'm so sorry, I shouldn't have merged it. I'm puzzled, I was overconfident because I had run a full local build w/o this problem.

I'll need to send another PR to experiment as I suspect the error is caused by interaction with the scripts we have for github actions.