[RELEASE PLUGIN] Guide for mvn artifact:compare

The commons release plugin includes this helper note:

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4b) Check reproducibility

To check that a build is reproducible, run:

mvn clean verify artifact:compare -DskipTests -Dreference.repo=
https://repository.apache.org/content/repositories/staging/
 '-Dbuildinfo.ignore=*/*.spdx.json'

Note that this excludes SPDX files from the check.
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However there are some caveats:

1. The timezone must match.
2. The JDK must match the one used for the release build.

I suggest the instructions be updated with this information, e.g.

---
# Use JDK 11
export TZ="Europe/London"
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I believe the timezone may be solved by always building our releases using
the TZ=UTC. But the correct JVM is still required.

Thoughts on this?

Alex