proc-macro-test: Pass target to cargo invocation

When cross compiling macos → dragonfly the dist build fails in the
proc-maro-test-impl crate with the following error:

ld: unknown option: -z\nclang: error: linker command failed with
exit code 1 (use -v to see invocation)

This appears to be a wart stemming from using an Apple host for cross
compiling.  Passing the target along to cargo allows it to pick up
a linker that it understands and DTRT.

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