uninstalls toolchains prior to deleting the rustup home folder

[Darunada]

Fixes #2789

I was able to reproduce the issue, and with this change the command now succeeds with the following output.

C:\Users\Lea>rustup self uninstall


Thanks for hacking in Rust!

This will uninstall all Rust toolchains and data, and remove
%USERPROFILE%\.cargo/bin from your PATH environment variable.

Continue? (y/N) y

info: removing toolchains
info: uninstalling toolchain 'nightly-x86_64-pc-windows-msvc'
info: toolchain 'nightly-x86_64-pc-windows-msvc' uninstalled
info: uninstalling toolchain 'stage2'
info: toolchain 'stage2' uninstalled
info: removing rustup home
info: removing cargo home
info: removing rustup binaries
info: rustup is uninstalled

[kinnison]

I'd like to see at least one test validating that this uninstall is happening. Probably it ought to be a new test in tests/cli-self-upd.rs something like an fn uninstall_removes_installed_toolchains()

[Darunada]

sure thing, work has gotten a bit more stressful in the last couple weeks, I will try to do something this weekend

[Rustin170506]

@Darunada Do you still have time to advance it? If you don't have time I would be happy to help add that test. But it would be great if you could move forward. Thanks!

[Darunada]

Sorry for the delay, I went ahead and added a test. Let me know if you like the approach!

[Rustin170506]

I think we don't have a lifetime argument here.