Add an env var which points to a config file for all pipelines

New feature

Currently there are several ways to provide a nextflow.config and I suggest allowing an extra one, an environment variable (e.g. NXF_CONFIG) which points to an arbitrary path.

Use case

This would allow the provision of a nextflow config in a location which is not writable by users since currently NXF_HOME / NXF_WORK must be writable to be functional. The only way to do this currently as far as I understand it is via a bash alias alias nextflow="nextflow -c {some_config}" which is quite fragile or adding the file to the users homedir which runs into the same problem as above, it also makes updating the config more complex and fragile due to how our homedirs work.

This would be handy for my specific usecase where we run a cluster in which users must use a specific nextflow config to make proper use of our cluster resources (k8s) and we don't want users to notice this or have the ability to break the config by modifying it.

Suggested implementation

Covered above I think.