selftests/vsock: fix vsock_test path shadowing in nested VMs

selftests/vsock: fix vsock_test path shadowing in nested VMs

The /root mount introduced for nested VM support shadows any host paths
under /root. This breaks systems where the outer VM runs as root and the
vsock_test binary path is something like:

/root/linux/tools/testing/selftests/vsock/vsock_test

Fix this by copying vsock_test into the temporary home directory that
gets mounted as /root in the guest, and using a relative path to invoke
it.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260317-vsock-vmtest-nested-fixes-v2-2-0b3f53b80a0f@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>