Auto merge of #20998 - matt-y:python-virtualenv-setup-pip-incovation-fix, r=jdm

Change to pip dependency install invocation during virtualenv setup in build

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Fix for https://github.com/servo/servo/issues/20990

In short, the pip invocation was causing my build on OSX to break. Using `python -m pip` in place of `pip` when setting up the virtual environment fixes the environment setup failure and allows the build to proceed.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #20990

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