Respect `NXF_HOME`

[ewels]

If you set the environment variable NXF_HOME, then some of the nf-core/tools features such as nf-core list do not work properly.

Here is the relevant code block in this example:

tools/nf_core/list.py

Lines 129 to 132 in b67fd2a

	if len(os.environ.get("NXF_ASSETS", "")) > 0:
	nextflow_wfdir = os.environ.get("NXF_ASSETS")
	else:
	nextflow_wfdir = os.path.join(os.getenv("HOME"), ".nextflow", "assets")

Need to also check for NXF_HOME as well as NXF_ASSETS (and any others?) and use that if set, before finally defaulting to the home directory path. Should also check the codebase for any other places where the same thing happens.