newapkbuild: fetch --target for rust

this fetches only things for the target triple, so it usually skips e.g.
all the windows crates.
we do this in aports already, in places where it doesn't work we just
unset it.

newapkbuild.in

@@ -32,7 +32,7 @@ is_github_url() {
 
 prepare_rust() {
 	cat >>APKBUILD<<__EOF__
-	cargo fetch --locked
+	cargo fetch --target="$CTARGET" --locked
 __EOF__
 }