Change v2 mac_unopt orchestrator builder to Linux (#41184)

`mac_unopt` orchestrator kicks off Mac swarming tasks, but don't seem to need to run on a Mac.  Swap Linux bots which are more plentiful and have a shorter queue time.

Start by adding new `bringup` builder to see if it passes in staging.  If it passes I will remove the Mac variant ASAP.

Led run: https://luci-milo.appspot.com/raw/build/logs.chromium.org/flutter/led/keyonghan_google.com/49f579a1636ddec2ffc05cb6053196a4472f5e4f69034386f88824356c479621/+/build.proto

Keep `mac_ios_engine` and `mac_host_engine` as Macs since they need to run Xcodes on Macs to create the xcframeworks.

See also #41181 and #41210

Author: jmagman

.ci.yaml

@@ -371,6 +371,14 @@ targets:
       add_recipes_cq: "true"
     timeout: 60
 
+  - name: Linux mac_unopt
+    bringup: true
+    recipe: engine_v2/engine_v2
+    properties:
+      config_name: mac_unopt
+      add_recipes_cq: "true"
+    timeout: 60
+
   - name: Mac Unopt
     bringup: true
     recipe: engine/engine_unopt