If you use the NRFUTIL_PACKAGE_INDEX_NAME environment variable with a non-existent modifier, nRF Util will
configure the modifier for you. This way, you can use a custom package index through environment variables.
For example:
NRFUTIL_PACKAGE_INDEX_NAME=custom-internal \
nrfutil +internal list
This command would create the internal modifier config entry, if this modifier does not already exist.
If an entry does exist, the value of NRFUTIL_PACKAGE_INDEX_NAME will take precedence over the modifier but the
modifier configuration will remain unchanged.