ci: move `ffi-check` docs build inside its build script

[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.

[davidhewitt]

With #5624 merged this particular PR is no longer needed, but I would like to return to this and merge the change to move docs build inside the build script. I'll leave as draft for now.