x11/squeekboard: Unbreak with CARGO_BUILD_TARGET in the environment (D31872)

cp: ./release/librs.a: No such file or directory
Traceback (most recent call last):

File "/wrkdirs/usr/ports/x11/squeekboard/work/squeekboard-8293c5f10dbd5c1fdb071e1f7e4a97a4cd52d592/cargo_build.py", line 45, in <module>
  subprocess.run(['cp', '-a',
File "/usr/local/lib/python3.8/subprocess.py", line 516, in run
  raise CalledProcessError(retcode, process.args,

subprocess.CalledProcessError: Command '['cp', '-a', './release/librs.a', PosixPath('/wrkdirs/usr/ports/x11/squeekboard/work/squeekboard-8293c5f10dbd5c1fdb071e1f7e4a97a4cd52d592/_build/src/librs.a')]' returned non-zero exit status 1.

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-09-09_14h16m24s/logs/errors/squeekboard-1.14.0_2.log

PR: 258337