PSA: avoid rustc 1.27 on windows

[Kartikaya Gupta]

The mozilla-central minimum rust requirement was recently bumped to
1.26 [1]. If you had an older rust version, you'll be forced to
update. But if you update your rust install via `rustup update stable`
you'll get 1.27, which has a known bug on Windows. This bug [2] will
manifest as boolean values not getting interpreted correctly across
the FFI boundary in some cases - right now if you build m-c with 1.27
on Windows this manifests as a panic in pop_stacking_context because
the aIsReferenceFrame bool isn't sent over correctly.

Firefox CI is using 1.28 beta so it doesn't run into this problem. For
local builds you can probably also use 1.28 beta or nightly or
something.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1447116
[2] https://bugs.llvm.org/show_bug.cgi?id=36886

[Kartikaya Gupta]

Actually even the latest rustc nightly has this problem, because rustc
nightlies have been blocked for a week due to an RLS issue. The fix
for the compilation error only landed after that. So if you're
building on Windows, 1.28 beta 6 (which has the fix uplifted) is the
best bet for getting a working version of firefox: `rustup update beta
&& rustup default beta && ./mach build`