Add clean to cherrypick.sh before runnign (precommit) check

[janhoy]

If not doing this, old build directories on disk may cause false test failures, such as detecting broken links in old stale javadoc, which is not a problem if re-built.

I had a cherry pick abort with below message, but turned out it was checking stale javadoc...

> Task :solr:documentation:checkBrokenLinks FAILED
Broken links check failed. First lines of the output:

Crawl/parse...

Verify...

file:///Users/janhoy/git/solr2/solr/documentation/build/site/modules/llm/overview-tree.html
  BROKEN LINK: file:///Users/janhoy/git/solr2/solr/documentation/build/site/core/org/apache/solr/search/neural/AbstractVectorQParserBase.html
  BROKEN LINK: file:///Users/janhoy/git/solr2/solr/documentation/build/site/core/org/apache/solr/search/neural/KnnQParser.html
[...]

[epugh]

I don't use the script everytime, but I do anytime it's more then the most minor change, and so this makes sense even if it makes things take longer! Cause I am using it for a more meaty backport.