Passing install args to WithPip doesn't override install args like withuv does

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

Our python packages are old and use old pip semantics for installation.
They run pip install with no arguments and let the setup.py pip installation flow take over.

WithUv args override the args totally but WithPip install args append and I don't see how I can remove the -r requirements.txt from it.

Also, I couldn't figure out how to get aspire to create the venv of the app - it was trying to use a pip.exe from a venv it didn't create yet.

### Expected Behavior

Aspire creates the venv using python -m venv .venv and allows overriding the pip install arguments.

### Steps To Reproduce

If needed I can create an example

### Exceptions (if any)

_No response_

### Aspire doctor output

_No response_

### Anything else?

_No response_