* guix/build-system/cargo.scm (cargo-triplet): New procedure.
(cargo-build, cargo-cross-build): Add cargo-target.
(lower): Add cargo-target as private keyword.
* guix/build/cargo-build-system.scm (configure): Use cargo-target to set
CARGO_BUILD_TARGET.
Change-Id: I476fa195dce0e9d20d59dda7dd15f050a56a15cb