[Bugfix] Pip - Access is denied durring installation (#3123)

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.

Author: lstein

installer/lib/installer.py

@@ -144,8 +144,8 @@ def app_venv(self, path: str = None):
 
             from plumbum import FG, local
 
-            pip = local[get_pip_from_venv(venv_dir)]
-            pip[ "install", "--upgrade", "pip"] & FG
+            python = local[get_python_from_venv(venv_dir)]
+            python[ "-m", "pip", "install", "--upgrade", "pip"] & FG
 
         return venv_dir
 
@@ -412,6 +412,22 @@ def get_pip_from_venv(venv_path: Path) -> str:
     return str(venv_path.expanduser().resolve() / pip)
 
 
+def get_python_from_venv(venv_path: Path) -> str:
+    """
+    Given a path to a virtual environment, get the absolute path to the `python` executable
+    in a cross-platform fashion. Does not validate that the python executable
+    actually exists in the virtualenv.
+
+    :param venv_path: Path to the virtual environment
+    :type venv_path: Path
+    :return: Absolute path to the python executable
+    :rtype: str
+    """
+
+    python = "Scripts\python.exe" if OS == "Windows" else "bin/python"
+    return str(venv_path.expanduser().resolve() / python)
+
+
 def set_sys_path(venv_path: Path) -> None:
     """
     Given a path to a virtual environment, set the sys.path, in a cross-platform fashion,