Add env variable to override the fallback settings file path

[frazar]

This PR introduces the support for overriding the path of the fallback settings file (/etc/rustup/settings.toml) by specifying the RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS environment variable.

The fix can be verified by running the following commands

$ # Create an `/etc/rustup/settings.toml` file specifying a default toolchain
$ echo "default_toolchain = 'stable'" | sudo tee /etc/rustup/settings.toml
default_toolchain = 'stable'
$ cargo test update_once # Fails!
$ export RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS=/dev/null 
$ cargo test update_once # Does not fail

Fixes #2456

Issue #2456 also mentions that the env variable should be "set up in the mock test support glue", but I'm not sure what it means..

[kinnison]

Thank you for trying to fix this issue.

There's a minor snafu in your code which will be easy for you to fix, however the important reason for wanting this feature is so that our test code doesn't use /etc/rustup/settings.toml when testing rustup. In here: https://github.com/rust-lang/rustup/blob/master/tests/mock/clitools.rs#L410 you will see how we set up the environment to run test code, at minimum setting the environment variable to something we know won't exist might be a good starting point. However this could also be an opportunity to introduce the ability to test the fallback code.

[kinnison]

While it pains me to say, you didn't format the code or test it before submitting that. There's a little more work you need to do, likely involving a bit of shifting around of the PathBuf::from and friends before it'll compile.

Also, have you given any thought to a way we might test this?

[frazar]

Sorry, I didn't mean to actually push this! It's still a work in progress, I'll update the PR title accordingly.

Thanks for your advices!

[frazar]

I think this PR is ready for a second review!

The unit test setup was updated to set the RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS variable with a bogus value, as suggested. This should make sure that the actual fallback settings file does not interefere with testing.

Also, a unit test was added for Unix plarforms where the fallback settings mechanism is tested.

Let me know if further changes are required e.g. squashing the commits.

[kinnison]

I think this looks good. @rbtcollins since this touches the process/env stuff I'd like you to just run your eye over it; but I think it's right.

[kinnison]

Content-wise it looks good. If @rbtcollins is okay with it, then I'd like it rebased into a neater commit if you don't mind.

[frazar]

Commits squashed! Let me know if anything else needs fixing

[kinnison]

I'm going to assume Robert is too busy right now, and so since I think it's good I'll merge it, thank you.