Honor environment variable CARGO

[NobodyXu]

According to Environment variables Cargo sets for 3rd party subcommands, cargo sets environment variable CARGO for subcommands.

detect-targets and cargo-binstall probably need to use this environment variable when present.

detec-targets needs to run $CARGO -vV, which also prints host: just as rustc -vV to stdout:

cargo 1.64.0 (387270bc7 2022-09-16)
release: 1.64.0
commit-hash: 387270bc7f446d17869c7f208207c73231d6a252
commit-date: 2022-09-16
host: aarch64-apple-darwin
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.79.1 (sys:0.4.55+curl-7.83.1 system ssl:(SecureTransport) LibreSSL/3.3.6)
os: Mac OS 12.6.0 [64-bit]

Where as cargo-binstall needs to run $CARGO install ....