Global requirements plugin: don't install by default on vf tmp

[HappyEinara]

• I have searched the issues (including closed ones) and believe that this is not a duplicate. [NB this relates to but does not duplicate Deactivate global requirements #69]
• I have searched the documentation and believe that my question is not covered.
• I am willing to lend a hand to help implement this feature.

Feature Request

I'm of the tentative view that VIRTUALFISH_GLOBAL_REQUIREMENTS should default to 0 in the case of a temporary venv created with vf tmp.

Almost always when I use a temp venv it's for some quick-and-dirty adhoc look at an issue with very specifically controlled packages and python versioning involved; I'm not sure of any other compelling use cases for temporary venvs. It would be a pleasant dev experience if a temp venv were pure by default, at the cost of a little documentation to avoid confusion.

And of course, this decision could be overridden with VIRTUALFISH_GLOBAL_REQUIREMENTS=1 in the individual case or at the project or user level for devs with that preference.

Feel free to close this if you disagree. An alternative approach is to use hooks to implement this behaviour for myself if I'm alone in this view, but I wanted to share the suggestion in case it would benefit others.