Mk/Uses/cargo.mk: Fix install for Rust 2018 edition applications

`cargo install` currently fails with:

error: Using `cargo install` to install the binaries for the package
in current working directory is no longer supported, use `cargo
install --path .` instead.

Also see rust-lang/cargo#5327


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@487583 35697150-7ecd-e111-bb59-0022644237b5

Author: Tobias Kortkamp

Mk/Uses/cargo.mk

@@ -251,6 +251,7 @@ do-build:
 .if !target(do-install) && ${CARGO_INSTALL:tl} == "yes"
 do-install:
 	@${CARGO_CARGO_RUN} install \
+		--path . \
 		--root "${STAGEDIR}${PREFIX}" \
 		--verbose \
 		${CARGO_INSTALL_ARGS}