Fix std_detect_env_override feature on Windows by avoiding libc

[dpaoliello]

As noted in #1563, the only dependency on libc when building stdarch for Windows was a call to getenv when the std_detect_env_override feature was enabled. Since the libc dependency is only enabled for non-Windows platforms, enabling the std_detect_env_override feature on Windows would cause a build break.

This change replaces the use of getenv with a call to GetEnvironmentVariableA on Windows and enables testing the std_detect_env_override feature.

Fixes #1563

r? @ChrisDenton

[rustbot]

Failed to set assignee to ChrisDenton: invalid assignee

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[ChrisDenton]

lgtm

Though it looks like CI is stuck? cc @Amanieu

[Amanieu]

I think your changes to the Github Actions configuration broke something.

[dpaoliello]

I think your changes to the Github Actions configuration broke something.

Ah, yep, I messed up the matrix syntax - sorry!

[dpaoliello]

@Amanieu CI has been fixed up - I also added --no-self-update to each of the rustup invocations since the self-update was causing issues with the Windows builds.