Instead of trying to use the user config dir, and then fail back to the
OS temp dir, just always use the temp dir. Also use a filename that is
less likely to cause collisions.
This addresses an issue on a test synology instance that was
mysteriously failing because there was a file at /tmp/tailscale. We
could still technically run into this issue if a
/tmp/tailscale-web-csrf.key file exists, but that seems far less likely.
Updates tailscale/corp#13775
Signed-off-by: Will Norris <will@tailscale.com>
37eab31f68