* guix/build/cargo-build-system.scm (configure): Add environment variables to use system libgit2 and libssh2. * gnu/packages/crates-io.scm (rust-libgit2-sys-0.10, rust-libssh2-sys-0.2)[arguments]: Remove LIBGIT2 and LIBSSH2 environment variable settings. * gnu/packages/rust-apps.scm (tokei)[arguments]: Same.