Run bridges zombienet tests on CI (#2439)

Brridges zombienet tests are non-standard - zombienet currently missing
multiple relay chains support (see e.g.
https://github.com/paritytech/zombienet/pull/796), so we need to go live
with two relay networks, their parachains + custom test runner (which
e.g. doesn't shutdown net when its tests are finished and instead waits
for both networks tests to complete). So we are stuck with native
zombienet provider => this PR is an attempt to gather everything in a
single docker container and run tests there ~Draft, because it is far
from finishing - what I want now is to see how it works on CI~

cumulus/scripts/bridges_rococo_westend.sh

@@ -132,10 +132,10 @@ LANE_ID="00000002"
 XCM_VERSION=3
 
 function init_ro_wnd() {
-    ensure_relayer
+    local relayer_path=$(ensure_relayer)
 
     RUST_LOG=runtime=trace,rpc=trace,bridge=trace \
-        ~/local_bridge_testing/bin/substrate-relay init-bridge rococo-to-bridge-hub-westend \
+        $relayer_path init-bridge rococo-to-bridge-hub-westend \
 	--source-host localhost \
 	--source-port 9942 \
 	--source-version-mode Auto \
@@ -146,10 +146,10 @@ function init_ro_wnd() {
 }
 
 function init_wnd_ro() {
-    ensure_relayer
+    local relayer_path=$(ensure_relayer)
 
     RUST_LOG=runtime=trace,rpc=trace,bridge=trace \
-        ~/local_bridge_testing/bin/substrate-relay init-bridge westend-to-bridge-hub-rococo \
+        $relayer_path init-bridge westend-to-bridge-hub-rococo \
         --source-host localhost \
         --source-port 9945 \
         --source-version-mode Auto \
@@ -160,10 +160,10 @@ function init_wnd_ro() {
 }
 
 function run_relay() {
-    ensure_relayer
+    local relayer_path=$(ensure_relayer)
 
     RUST_LOG=runtime=trace,rpc=trace,bridge=trace \
-        ~/local_bridge_testing/bin/substrate-relay relay-headers-and-messages bridge-hub-rococo-bridge-hub-westend \
+        $relayer_path relay-headers-and-messages bridge-hub-rococo-bridge-hub-westend \
         --rococo-host localhost \
         --rococo-port 9942 \
         --rococo-version-mode Auto \