Add TESTNET_TAG Env var to buildkite (#3692)

* Add TESTNET_TAG Env var to buildkite

ci/testnet-manager.sh

@@ -75,6 +75,9 @@ source scripts/configure-metrics.sh
 ci/channel-info.sh
 eval "$(ci/channel-info.sh)"
 
+if [[ -n $TESTNET_TAG ]]; then
+  CHANNEL_OR_TAG=$TESTNET_TAG
+else
   case $TESTNET in
   testnet-edge|testnet-edge-perf)
     CHANNEL_OR_TAG=edge
@@ -113,7 +116,7 @@ EOF
     ) | buildkite-agent pipeline upload
     exit 0
   fi
-
+fi
 
 sanity() {
   echo "--- sanity $TESTNET"