Include package props with env vars into target metadata

Include package props with 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 such properties (there are 3 of them in total:
authors, description and homepage) in the target metadata.

Fixes #3696.