Use NXF_HOME env var for nextflow home directory, if set

[ewels]

Look for and use $NXF_HOME for stuff if set, before defaulting to ~/.nextflow

Closes #798

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[KevinMenden]

Looks good to me.
Couldn't find any other uses of NXF_HOME or NXF_ASSETS in the codebase. Except in test_list.py, but those are fine.
As far as I can tell, all the other Nextflow environment variables don't matter for this purpose.
Should be fine :)