Fix an issue where the cargo-extract target fails when a port sets DIST_SUBDIR.

PR: 275634

Note: This is my first commit touching Mk/. I'd suggest reviewing this extra carefully :)
My shell scripting skills are also below-average.

I have tested this with the following script:

#!/bin/sh

PORTS_DIR=/usr/ports
for d in $(portgrep -u cargo -o); do
	echo "Testing port: " $d
	dir=$PORTS_DIR/$d
	make -C $dir extract || echo $d >> failures.lst
done