fix(test): strip RUSTUP_TOOLCHAIN_SOURCE from test environment (#16857)

While I was trying to run the testsuite earlier, I kept running into
issues with `install` tests failing due to `cargo` warning about
toolchain overrides, as I have an override for my whole Rust projects
folder. After looking into it for a bit, I found that removing
`RUSTUP_TOOLCHAIN_SOURCE` from the `test_env` resolved the warnings
without causing any other tests to fail. I spent some time looking at
#16131 and #16469, trying to find ways this could cause problems, and
while I couldn't find any, I am not very confident in my assessment.
@smoelius do you have any thoughts?

Author: weihanglo

crates/cargo-test-support/src/lib.rs

@@ -1458,6 +1458,7 @@ pub trait TestEnvCommandExt: Sized {
             .env_remove("RUSTDOC")
             .env_remove("RUSTDOCFLAGS")
             .env_remove("RUSTFLAGS")
+            .env_remove("RUSTUP_TOOLCHAIN_SOURCE")
             .env_remove("SSH_AUTH_SOCK") // ensure an outer agent is never contacted
             .env_remove("USER") // not set on some rust-lang docker images
             .env_remove("XDG_CONFIG_HOME") // see #2345