Scope:

• Adding URLs that reference indygreg freethreaded toolchains. (feat(toolchain): support freethreaded toolchains #2372)
• Running coverage using freethreaded toolchains. (feat(toolchain): support freethreaded toolchains #2372)
• Depending on whls when freethreaded toolchains are used. (feat(pypi): support freethreaded in experimental_index_url #2460)
• Using a freethreaded toolchain interpreter in pip.parse rules to build from sdist. (refactor!(toolchain): remove uname dep in the repository_rule stage #2406).

Better support for sdist building should be done separately.