Mk/Uses/cargo.mk: Fix cargo-extract target

[jbo]

Summary

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

PR: 275634

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

Test Plan

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	

[tcberner]

lgtm as mentor, but I defer to rust for the context knowledge :D

[tobik]

Same general problem as D39205. In https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275922 you can see it in distinfo too where crates that should go into DISTDIR/rust/crates now go into DISTDIR/jetbrains/rust/crates:

TIMESTAMP = 1702039441
SHA256 (jetbrains/rust/crates/autocfg-1.1.0.crate) = d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa
SIZE (jetbrains/rust/crates/autocfg-1.1.0.crate) = 13272
[...]

But that shouldn't block committing this patch as is.