Work around RA's RUSTC_WRAPPER poisoning all subsequent command-line builds

[dtolnay]

Previously if someone did a build via rust-analyzer, it would run anyhow's build script through rust-analyzer's questionable RUSTC_WRAPPER (#250), and then future cargo invocations done on the command line not via rust-analyzer would bypass the build script, until cargo clean or the target dir being deleted.

This PR is an alternative to #251 that tries to isolate command-line cargo builds from getting poisoned by rust-analyzer's running of the build script and cargo's caching of the build script output. In contrast to #251, this does not attempt to fix builds done through rust-analyzer, only direct cargo invocations. I still consider rust-analyzer's behavior incorrect so I want to not sweep it under the rug; getting that fixed in rust-analyzer will be between the rust-analyzer devs and its users.

[ErichDonGubler]

CC @matklad.

[dtolnay]

I am also working on a Cargo PR to make this the default behavior in Cargo i.e. include RUSTC_WRAPPER in the fingerprint used for deciding whether to rerun every build script, as is already done for RUSTC and various other important environment details.