refactor!(toolchain): remove uname dep in the repository_rule stage

[aignas]

Before this PR we would shell out to uname on UNIX systems to get the arch
of the toolchain - on Windows we would not need to do it because there used to
be only a single Windows platform. With this change we can correctly support
the resolution of the python interpreter on various platforms and I have also
added an env variable to customize the selection, so that users can use musl
or a freethreaded interpreter if they wish.

As part of this change, I have restricted visibility of the config settings
used in the toolchain alias repo so that we are creating fewer targets. This is
a very good time to do this before 1.0.0.

Fixes #2145
Work towards #2276
Work towards #2386
Work towards #1211 to unblock #2402
Work towards #1361