version-helper: Also try parsing release date from Cargo.toml

The package.metadata.gstreamer.release_date date string can be used to specify the release date.

This is used if there's no git repository as a fallback before using the mtime of the Cargo.toml. Using the mtime will fail when building a crate packaged by cargo because cargo sets the mtimes to bogus values.

Fixes #440 (closed)