Append Windows "bin" directory to PATH

[ChrisDenton]

Hopefully fixes #3825

On Windows rustc's "bin" directory is now appended to the end of PATH so that:

• All DLLs within "bin" can be found by the loader but
• The EXEs within "bin" will not override anything else on PATH

I can't think of a downside to doing this but I'm happy to be corrected. As with prepending paths, it won't add to PATH if that already contains the path. This also means that the behaviour of RUSTUP_WINDOWS_PATH_ADD_BIN=1 is currently unchanged.

[djc]

I think if the env var is enabled this adds two path entries, is that desirable? Maybe we can ignore the env var after this change?

[djc]

LGTM but I defer judgement to people who better understand Windows stuff.

Maybe r? @wesleywiser?

[ChrisDenton]

i think if the env var is enabled this adds two path entries, is that desirable? Maybe we can ignore the env var after this change?

The deduplication logic should stop the path being added twice. But I have no objection to removing the env var.

[ChrisDenton]

The PATH behaviour can now be fully controlled using the RUSTUP_WINDOWS_PATH_ADD_BIN environment variable:

• RUSTUP_WINDOWS_PATH_ADD_BIN=0 is the 1.27 behaviour of not adding the bin directory to PATH
• RUSTUP_WINDOWS_PATH_ADD_BIN=1 is the older behaviour of prepending the bin directory to PATH
• Unset or anything else appends the bin directory to PATH

[djc]

Changes LGTM! I think the current commit history doesn't totally makes sense, maybe just squash the changes so there's only one commit?

[wesleywiser]

Makes sense to me but I agree that making this new behavior controlled by RUSTUP_WINDOWS_PATH_ADD_BIN=2 and that the default would be more cautious.