improve reproducibility of the build

[hboutemy]

as checked in Reproducible Central https://github.com/jvm-repo-rebuild/reproducible-central#com.taobao.arthas:arthas-all, reproducibility of latest releases has improved (with 3.4.4 having been done from a dirty Git workspace...) but after having checked 3.4.5, there are 2 remaining issues:

1. there are 2 remaining non-reproducible Git properties, that this PR removes
2. something during the rebuild changes pom.xml xsd from http to https

Changing xsd to https://maven.apache.org/xsd/maven-4.0.0.xsd in source tree could be a good idea, I don't know if there are any objections

[hengyunabc]

Thank you very much, the xsd urls changed to https.

[hboutemy]

FYI, I just understood what changed http to https (or vice versa): it's flatten-maven-plugin, that changes the value based on Maven version (because of https://issues.apache.org/jira/browse/MNG-6778 )

then:

1. it's better to have https in your sources
2. I'll update my reproduce script to use mvnw, to make sure the reproduce script uses the same Maven version than you