DOC: Document location in error message

[larsoner]

Consider this an PR-as-suggestion for discussion. Locally on my macOS machine where I use conda I just ran:

$ python --version
Python 3.13.5
$ CIBW_BUILD="cp313-*" cibuildwheel .
...
cibuildwheel: error: Error: CPython 3.13 is not installed.
cibuildwheel will not perform system-wide installs when running outside of CI.
...

I found this error message to be confusing, since I do have Python 3.13 installed. This PR suggests an update to the message to:

cibuildwheel: error: Framework CPython 3.13 not detected as installed in:
    /Library/Frameworks/Python.framework/Versions/3.13
cibuildwheel will not perform system-wide installs when running outside of CI.
...

This would have made it clear that my conda-provided 3.13 isn't sufficient, it needs to be a framework version. It also removes the duplicate "Error: ".

[henryiii]

I think this looks fine. @joerick?