[ci] Switch master Linux custom package tests to LUCI (#4386)

Enables the new LUCI target for `master`, and removes the Cirrus version.

I only pre-tested `master`, and it turns out that `stable` is blocked by flutter/flutter#130071.

Part of flutter/flutter#114373

.ci.yaml

@@ -211,7 +211,6 @@ targets:
       version_file: flutter_stable.version
 
   - name: Linux_android custom_package_tests master
-    bringup: true # New target
     recipe: packages/packages
     timeout: 30
     properties:
@@ -228,7 +227,7 @@ targets:
       channel: master
 
   - name: Linux_android custom_package_tests stable
-    bringup: true # New target
+    bringup: true # Blocked on https://github.com/flutter/flutter/issues/130071
     recipe: packages/packages
     timeout: 30
     properties:

.cirrus.yml

@@ -161,9 +161,9 @@ task:
     - name: linux-custom_package_tests
       env:
         PATH: $PATH:/usr/local/bin
-        matrix:
-          CHANNEL: "master"
-          CHANNEL: "stable"
+        # Master has been migrated to LUCI, but stable is blocked on
+        # https://github.com/flutter/flutter/issues/130071
+        CHANNEL: "stable"
       << : *INSTALL_CHROME_LINUX
       local_tests_script:
         - ./script/tool_runner.sh custom-test