Add check to nvm profile for nvm path

[sladyn98]

This PR aims to add a check for the profile before adding the path Closes https://github.com/nvm-sh/nvm/issues/2076

[sladyn98]

 ✓ nvm_check_global_modules
  ✓ nvm_detect_profile
  ✓ nvm_do_install
  ✓ nvm_install_dir
  ✓ nvm_install_with_aliased_dot
  ✓ nvm_install_with_node_version
  ✓ nvm_profile_is_bash_or_zsh
  ✓ nvm_reset
  ✓ nvm_source

Looking at the travis I see that the nvm_check_if_path_exists test does not run here

[ljharb]

ah, you'll need to chmod a+x test/install_script/nvm_check_if_path_exists so that it can be executed :-)

[ljharb]

I've rebased this, and marked it as a draft. Please mark it as ready for review when you think it's ready.

[sladyn98]

@ljharb Do you want me to complete this ?

[ljharb]

@sladyn98 that'd be ideal :-) but if you're not interested then it's fine to leave it open so someone else can pick it up.