Auto merge of #3857 - antonlarin:rebuild-on-env-change, r=alexcrichton

Include package props with corresponding env vars into target metadata

Previously, when changing package properties with corresponding environment variables (such as authors, which has CARGO_PKG_AUTHORS), it didn't invalidate the build, even though there could have been a dependency on such variables in the source code.

This commit includes 3 such properties: authors list, description and homepage in the target metadata.

I've added a test only for description change, can add more if necessary.
Fixes #3696.