Env var configuration errors may not always report the env var

[alexcrichton]

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 Target

Note 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!