Update odic-funnel-clients.json to take a path, this
allows setting the location of the file and prevents
it from landing in the root directory or users home directory.
Move setting of rootPath until after tsnet has started.
Previously this was added for the lazy creation of the
oidc-key.json. It's now needed earlier in the flow.
Updates #16734Fixes#16844
Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>