only-script for Linux clobbers MVNW_REPOURL

### Affected version

3.3.3

### Bug description

Running the Maven Wrapper plugin through the Maven Wrapper's `only-script` distribution type causes `MVNW_REPOURL` to be discarded, leaving the `distributionUrl` written to `maven-wrapper.properties` at its default value. Consequently, `only-script` for Linux is unable to update itself in any environment that needs `MVNW_REPOURL`.

That is, the hypothetical execution

```sh
$ MVNW_REPOURL=asdfasdf ./mvnw org.apache.maven.plugins:maven-wrapper-plugin:3.3.3:wrapper -Dtype=only-script -Dmaven=3.9.11 -X
```

where `asdfasdf` is a functional mirror should have produced an output similar to

```
...
[DEBUG] Using repo URL from MVNW_REPOURL environment variable.
[DEBUG] Determined repo URL to use as asdfasdf
[INFO] Configuring .mvn/wrapper/maven-wrapper.properties to use Maven 3.9.11 and download from asdfasdf
```

but will end up producing

```
...
[DEBUG] Determined repo URL to use as https://repo.maven.apache.org/maven2
[INFO] Configuring .mvn/wrapper/maven-wrapper.properties to use Maven 3.9.11 and download from https://repo.maven.apache.org/maven2
```

and the result is consistent with the log messages.

This happens because `only-mvnw` explicitly unsets `MVNW_REPOURL`:

https://github.com/apache/maven-wrapper/blob/maven-wrapper-3.3.3/maven-wrapper-distribution/src/resources/only-mvnw#L149

so by the time `WrapperMojo` tries to read the variable it sees `null`:

https://github.com/apache/maven-wrapper/blob/maven-wrapper-3.3.3/maven-wrapper-distribution/src/resources/only-mvnw#L149

As far as I can see, this bug does not exist in `only-mvnw.cmd`.

The issue is exacerbated by the user property `distributionUrl` value representing the "final" URl. That is, it is not simply a domain substitution and therefore cannot take the place of `MVNW_REPOURL`.