ci: remove ffi-check dependency on extension-module feature

[davidhewitt]

To unblock #5588

I made two changes:

• Use PYO3_BUILD_EXTENSION_MODULE env var to control pyo3-ffi-check linking (instead of the feature)
• Move the pyo3-ffi docs build inside the pyo3-ffi-check build script, makes it work a bit better.

With this, the pyo3-ffi-check functionality is better encapsulated within itself rather than in the noxfile.

[Icxolu]

I generally dislike to check-in the cargo config file. At least to some degree it contains options that are specific to the build machine which I might want to adjust locally just for myself.

[davidhewitt]

Fair concern. I'm beginning to think that the "right" solution is to fix the link issues rather than find new hacks to suspend it.