feat(cli): allow specifying an alternative cargo binary via CARGO env var

[messense]

This opens doors for using cargo-xwin to cross compiling for Windows MSVC targets, for example

CARGO=cargo-xwin yarn build --target x86_64-pc-windows-msvc
   Compiling winapi v0.3.9
   Compiling pin-project-lite v0.2.7
   Compiling futures-sink v0.3.19
   Compiling slab v0.4.5
   Compiling pin-utils v0.1.0
   Compiling futures-io v0.3.19
   Compiling ryu v1.0.9
   Compiling once_cell v1.9.0
   Compiling num_cpus v1.13.1
   Compiling cfg-if v1.0.0
   Compiling itoa v1.0.1
   Compiling lazy_static v1.4.0
   Compiling futures-core v0.3.19
   Compiling futures-task v0.3.19
   Compiling libc v0.2.112
   Compiling memchr v2.4.1
   Compiling futures-channel v0.3.19
   Compiling serde v1.0.132
   Compiling futures-util v0.3.19
   Compiling encoding_rs v0.8.30
   Compiling serde_json v1.0.73
   Compiling thread_local v1.1.4
   Compiling tokio v1.15.0
   Compiling num-traits v0.2.14
   Compiling num-integer v0.1.44
   Compiling napi-examples v0.1.0 (/Users/messense/Projects/napi-rs/examples/napi)
   Compiling libloading v0.7.3
   Compiling time v0.1.44
   Compiling futures-executor v0.3.19
   Compiling napi-sys v2.2.2 (/Users/messense/Projects/napi-rs/crates/sys)
   Compiling futures v0.3.19
   Compiling chrono v0.4.19
   Compiling napi v2.4.3 (/Users/messense/Projects/napi-rs/crates/napi)
    Finished dev [unoptimized + debuginfo] target(s) in 11.33s
file index.node
index.node: PE32+ executable (DLL) (GUI) x86-64, for MS Windows

And cargo-zigbuild can also be used the same way.

cc #1180