test the Nupm installation

[amtoine]

after talking with @stormasm about the new nu-git-manager / nu-git-manager-sugar install, i thought we could try to test the Nupm install inside our CI.

this PR is this experiment:

• install Nupm with itself in an extra CI step
• use ~/.local/share/nupm/modules/nupm as the standard location for the nupm package
• add the install-package test which
  • installs nu-git-manager to the Nupm store
  • checks that there is no scripts
  • checks that there are the two nu-git-manager and nu-git-manager-sugar modules installed

Note
currently there is no --no-confirm option for nupm install, so the NUPM_HOME has to be created manually with mkdir prior to the install => i've added FIXMEs for that
should be fixed with nushell/nupm#42

[melMass]

🚨 Super cool