[Bugfix] Pip - Access is denied durring installation

[StAlKeR7779]

Now, for python 3.9 installer run upgrade pip command like this:
pip install --upgrade pip
And because pip binary locked as running process this lead to error(at least on windows):

ERROR: Could not install packages due to an OSError: [WinError 5] Access is denied: 'e:\invokeai\.venv\scripts\pip.exe'
Check the permissions.

To prevent this recomended command to upgrade pip is:
python -m pip install --upgrade pip
Which not locking pip file.

[lstein]

This looks good! Thanks for the fix.