Added --pip-path argument to develop

[mitsuhiko]

This changes develop so that --pip-path can be passed to use a different pip. Example with rye:

maturin develop --pip-path ~/.rye/self/bin/pip

Fixes #1752

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[konstin]

One thing to check: maturin will pick up a folder called .venv even if VIRTUAL_ENV is not set (basically what PEP 704 used to propose). This works when running .venv/bin/pip, but if we run an external pip, will it pick up the right venv?

[mitsuhiko]

@konstin if i understand the logic correctly the picking up of the venv happens and this is passed as --python, no?

[konstin]

yes, i expect this to work, but we should add an automated test to ensure it does, especially across platforms.

[messense]

I'm ok with accepting this as-is since most end users don't need to pass --pip-path.