Clear CARGO environment variable in cargo proxy

[smoelius]

Fixes rust-lang/clippy#15099

To recount the problem (copied from rust-lang/cargo#15099 (comment)):

• The cargo proxy runs, selects the default cargo, and sets the CARGO environment variable to default toolchain's cargo.
• That cargo invocations runs some other code, which invokes cargo +<toolchain> <subcommand>.
• <subcommand> reads the CARGO environment variable and runs the default toolchain's cargo rather than <toolchain>'s cargo.

This PR fixes the problem by clearing the CARGO environment variable in the proxy when cargo +<toolchain> <subcommand> is run.

This PR is currently organized as eight commits:

• The first commit applies the fix.
• The next five commits add infrastructure to support the second-to-last commit.
• The second-to-last commit adds a test to address all scenarios in $CARGO passed to external subcommands can point to the wrong cargo if already set cargo#15099 (comment).
• The final commit is a small bug fix to Config::cmd.

Nits are welcome.