selftests: drv-net: use env.rpath in the HDS test

Commit 29b036be ("selftests: drv-net: test XDP, HDS auto and
the ioctl path") added a new test case in the net tree, now that
this code has made its way to net-next convert it to use the env.rpath()
helper instead of manually computing the relative path.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250228212956.25399-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>