[MNG-8156] mvn shell script should print the environment variables when “The JAVA_HOME environment variable is not defined correctly”

**[Didier Loiseau](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=didier%20loiseau)** opened **[MNG-8156](https://issues.apache.org/jira/browse/MNG-8156?redirect=false)** and commented

When `$JAVA_HOME` is not properly set, it just prints:

```
The JAVA_HOME environment variable is not defined correctly,
this environment variable is needed to run this program.
```

and stops.

This is not very helpful as there is absolutely no indication of what is wrong exactly and how to fix it. For instance, in [this Stack Overflow question](https://stackoverflow.com/q/78614953/525036), the user had a correct `$JAVA_HOME`, but it was unknowingly overridden in their `.mavenrc`

The command should at least print the values of `$JAVA_HOME` and `$JAVACMD`, and ideally some additional help. Something like:

> JAVA_HOME: $JAVA_HOME
> JAVACMD: $JAVACMD
> If this is not what you expect, you should check the environment variables, the mavenrc file(s), and ensure that you have execution rights on the java executable.
> You can try setting environment variable MAVEN_SKIP_RC=true to bypass mavenrc temporarily.

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**Affects:** 3.9.7