ocivm: Unify Portage cache directories

It turns out that QEMU microvms have a very limited number of I/O ports
available for e.g. attached disks and shared filesystems.  To conserve
ports, we'll use a single volume for both distfiles and binpkgs cache
directories.

ocivm.sh

@@ -15,8 +15,7 @@ ocivm \
     -n ${name} \
     -w /tmp/src \
     -v "${PWD}":/tmp/src \
-    -v distfiles:/var/cache/distfiles:size=4G \
+    -v portagecache:/var/cache:size=16G \
-    -v packages:/var/cache/binpkgs \
     -v portage:/var/db/repos/gentoo:size=4G \
     --tmpfs /usr/aarch64-unknown-linux-gnu/tmp \
     --tmpfs /var/tmp \