misc (Makefile): Fix custom venv treatment in Makefile

[compor]

This PR:

• Removes export from UV_PROJECT_ENVIRONMENT, according to here, its function is rather peculiar
• Adds a missed phony target

My issue is that doing:

1. VENV_DIR=.vevn make venv
2. source .venv/bin/activate
3. make tests

issues the following warning:

warning: VIRTUAL_ENV=.venv does not match the project environment path venv and will be ignored

and leads to failed tests (cannot find riscemu), as uv run according to its documentation will create a venv:

When used in a project, the project environment will be created and updated before invoking the command.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.15%. Comparing base (056f76e) to head (a924b51).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3626   +/-   ##
=======================================
  Coverage   90.14%   90.15%           
=======================================
  Files         463      463           
  Lines       58720    58720           
  Branches     5627     5627           
=======================================
+ Hits        52935    52937    +2     
+ Misses       4346     4345    -1     
+ Partials     1439     1438    -1     

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[superlopuh]

I've been wrangling with this locally, thanks for fixing it