This is simply an extra check to prevent hypothetical issues if a prefix
such as `--prefix="javascript:alert(1)"` was provided. This isn't
really necessary since the prefix is a configuration flag provided by
the device owner, not user input. But it does enforce that we are
always interpreting the provided value as a path relative to the root.
Fixes: tailscale/corp#16268
Signed-off-by: Will Norris <will@tailscale.com>
569b91417f