fix Nushell tests

because this was developed when Nushell was in 0.95.0, some tweaks had to be added

• allow to set the Nushell binary with the NU var in Makefile, e.g. make NU="nu-0.95.0" test if you have Nushell 0.95.0 installed as nu-0.95.0 in PATH
• use $nu.current-exe for nested Nushell invocation to preserve the version set in the top-level Makefile
• use --no-config-file to make sure there is no interaction with the user's config, which might not be for the correct version