Uses/cargo: Improve URL subsitution for ports using CARGO_USE_GIT[HUB|LAB]

While maintaing a rust application, I found a cargo.toml that specifies
the git URL with { git="https://..." }, cargo.mk expects a form like { git = "https://" }.
This patch improve the regex making the spaces around the '=' optional
Test: built all ports using the CARGO_USE_GIT feature

Approved by: tobik
Differential Revision: https://reviews.freebsd.org/D29805