docs: flip TOML and envvars order

[henryiii]

To start, I've asked Copilot for a script to flip the order of the tabs. It gave me this one:

import re

# filepath: /Users/henryschreiner/git/software/cibuildwheel/docs/options.md
file_path = "/Users/henryschreiner/git/software/cibuildwheel/docs/options.md"

# Regular expression to match the examples section
example_pattern = re.compile(
    r"(!!! tab examples \"Environment variables\".*?```.*?```.*?)"
    r"(!!! tab examples \"pyproject\.toml\".*?```.*?```.*?)",
    re.DOTALL
)

def flip_examples(file_path):
    with open(file_path, "r") as file:
        content = file.read()

    # Swap the order of the examples
    updated_content = example_pattern.sub(r"\2\n\n\1", content)

    with open(file_path, "w") as file:
        file.write(updated_content)

    print("Examples flipped successfully!")

# Run the function
flip_examples(file_path)

This commit is the result of that.

---

Next I'd like to update the headings so that both options are shown. I'd like to put one on the left, the other on the right, but not sure how to do that yet. Putting this here for now.