install/podstorage: Fix leak of overlay mount

[cgwalters]

Reworking the test suite I see

 ---- Install and verify selinux state ----
No SELinux label found for: "bootc/storage/overlay-layers/f8535836adcfdf205e158fa9cc22844629c78d38940edf6b2a264a013bb7240b.tar-split.gz"

I think this is because something in podstorage.rs is leaking a mount point for bootc/storage (c/storage makes one) and this is causing the recursive SELinux relabeling traversal to skip it.