Env var configuration errors may not always report the env var #7781
Description
Playing around a bit today, I manage to get a good error (yay!)
$ CARGO_BUILD_PIPELINING=not-a-bool cargo +nightly build
error: error in environment variable `CARGO_BUILD_PIPELINING`: provided string was not `true` or `false` but then playing some more I got a different error:
$ CARGO_BUILD_RUSTFLAGS=true cargo +nightly build
error: failed to deserialize, expected a string or array of strings: data did not match any variant of untagged enum TargetNote that the error in the latter one may be attributable to #7780, but despite that maybe being a bug it's probably also a bug that the env var isn't mentioned!