index.cgi: pass path prefix to web CLI

prefix flag added in tailscale/tailscale#9189, and needed for new web
client.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>

Author: willnorris

Tailscale/shared/ui/index.cgi

@@ -2,4 +2,4 @@
 CONF=/etc/config/qpkg.conf
 QPKG_NAME="Tailscale"
 QPKG_ROOT=$(/sbin/getcfg ${QPKG_NAME} Install_Path -f ${CONF} -d"")
-exec ${QPKG_ROOT}/tailscale --socket=/tmp/tailscale/tailscaled.sock web -cgi
+exec "${QPKG_ROOT}/tailscale" --socket=/tmp/tailscale/tailscaled.sock web --cgi --prefix="/cgi-bin/qpkg/Tailscale/index.cgi/"