net-test: buster -> bullseye

as aosp master has upgraded to iptables 1.8.4 and
buster image simply doesn't build any more...

(there is no debian 1.8.4 for buster [stable],
 but there is for bullseye [testing])

Tested: known to not work, see followup commit
  find | egrep buster
  git grep buster
  (finds nothing)
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I01b973367354e12d237f53b593b768fdcb4fde54
diff --git a/net/test/build_rootfs.sh b/net/test/build_rootfs.sh
index 2f58a89..d31674d 100755
--- a/net/test/build_rootfs.sh
+++ b/net/test/build_rootfs.sh
@@ -27,14 +27,14 @@
 export LC_ALL=C
 
 usage() {
-  echo -n "usage: $0 [-h] [-s buster] [-a i386|amd64|armhf|arm64] "
+  echo -n "usage: $0 [-h] [-s bullseye] [-a i386|amd64|armhf|arm64] "
   echo "[-m http://mirror/debian] [-n net_test.rootfs.`date +%Y%m%d`]"
   exit 1
 }
 
 mirror=http://ftp.debian.org/debian
 debootstrap=debootstrap
-suite=buster
+suite=bullseye
 arch=amd64
 
 while getopts ":hs:a:m:n:" opt; do
@@ -43,7 +43,7 @@
       usage
       ;;
     s)
-      if [[ "$OPTARG" != "buster" ]]; then
+      if [[ "$OPTARG" != "bullseye" ]]; then
         echo "Invalid suite: $OPTARG" >&2
         usage
       fi