install-deps.sh: do not use deb.rug.nl as a mirror anymore

install-deps.sh: do not use deb.rug.nl as a mirror anymore

seems it does not mirror bionic packages for ubuntu-toolchain-r PPA repo anymore.
see http://deb.rug.nl/ppa/mirror/ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu/dists/

without this change, `sudo apt-get update` fails like:

Err:5 http://deb.rug.nl/ppa/mirror/ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu bionic Release
  404  Not Found [IP: 129.125.36.99 80]
...

which fails the build which runs install-deps.sh, like:

Build step 'Execute shell' marked build as failure

Signed-off-by: Kefu Chai <kchai@redhat.com>