Allow unsetting environment variables

[nickbabcock]

Just has support for setting environment variables:

export RUST_BACKTRACE := "1"

test:
    cargo test

But does not support unsetting environment variables

unset RUST_BACKTRACE

test:
    cargo test

In the ideal scenario, the environment variable has an equivalent value to being unset (0 in the case for RUST_BACKTRACE)

Unfortunately, some programs, crates, and/or libraries treat an unset environment variable different than even an empty string, so it would be helpful to be able to unset these at the global level instead of remembering to sprinkle in the manual method everywhere:

test:
    #!/bin/bash -x
    unset RUST_BACKTRACE
    cargo test

[casey]

This definitely seems useful, and the unset FOO seems reasonable. The best way to implement this is probably with a new Item variant, Item::Unset(String), which causes exported variables to be unset.

[neunenak]

Should be closeable now with #2098