Docs: Info about CARGO_ZIGBUILD_PYTHON_PATH

[mert-kurttutan]

One of machines had python binary named python (instead of python3). I looked into source code and discovered CARGO_ZIGBUILD_PYTHON_PATH, then used it, cargo zigbuild worked properly.

Also, the error due to not-finding python is reported as

Error: Failed to find zig

Caused by:
    cannot find binary path

Two suggestions:

• I think this particular error (related python binary) should be reported as failed to find python.
• Add documentation about env var CARGO_ZIGBUILD_PYTHON_PATH (I was not able to find docs about this)

[mert-kurttutan]

I am willing to make PR, if necessary.