cargo fix doesn't work with RUSTC_WRAPPER=sccache

[dwijnand]

Before:

$ cargo +nightly new --lib foo && command cd foo && cargo +nightly fix --lib
     Created library `foo` project
    Checking foo v0.1.0 (file:///Users/dnw/Desktop/foo)
error: failed to execute compile
caused by: Bad relative path
error: Could not compile `foo`.

After:

$ unset RUSTC_WRAPPER; cargo +nightly new --lib foo && command cd foo && cargo +nightly fix --lib
     Created library `foo` project
    Checking foo v0.1.0 (file:///Users/dnw/Desktop/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.82s

[alexcrichton]

Oh dear sounds bad! I think for cargo fix this is basically incompatible right now, so we should probably just forcibly disable handling of RUSTC_WRAPPER