rustc: Request ansi colors if stderr isn't a tty

Currently Cargo will always capture the output of rustc meaning that
rustc is never hooked up to a tty. To retain colors Cargo uses the
`fwdansi` crate to ensure that ansi color codes are translated to
windows terminal methods (and ansi codes otherwise just go their natural
route on Unix).

Cargo passes `--color always` to rustc to ensure that using a pipe
doesn't trick it into not emitting colors at all. It turns out, however,
that `--color always` ends up still accidentally using the native shell
api on native windows shells.

The fix here is to instead pass `AlwaysAnsi` to `termcolor` instead of
`Always`, ensuring that when `--color always` is passed to rustc and its
output isn't a terminal, we're always generating ansi colors regardless
of the platform.

Closes #55769

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