devel/py-maturin: Unbreak with CARGO_BUILD_TARGET in the environment (D31872)

error: can't copy '/wrkdirs/usr/ports/devel/py-maturin/work-py38/maturin-0.8.3/target/release/maturin': doesn't exist or not a regular file

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-09-09_14h16m24s/logs/errors/py38-maturin-0.8.3_8.log

PR: 258337