[ci] Pin master for CI tests (#554)

Updates to match the recent change in flutter/plugins, to use a pinned
version of master instead of the latest version. This uses the last
version of master that passed post-submit tests due to a current
out-of-band failure in golden tests.

This affects both the Cirrus tests and the Windows LUCI tests.

Part of https://github.com/flutter/flutter/issues/93811

.ci.yaml

@@ -36,6 +36,7 @@ targets:
       add_recipes_cq: "true"
       target_file: windows_local_tests.yaml
       channel: master
+      version_file: flutter_master.version
     scheduler: luci
 
   - name: Windows win32-platform_tests master - packages
@@ -45,6 +46,7 @@ targets:
       add_recipes_cq: "true"
       target_file: windows_build_and_platform_tests.yaml
       channel: master
+      version_file: flutter_master.version
       dependencies: >
         [
           {"dependency": "vs_build"}

.cirrus.yml

@@ -9,15 +9,25 @@ tool_setup_template: &TOOL_SETUP_TEMPLATE
 
 flutter_upgrade_template: &FLUTTER_UPGRADE_TEMPLATE
   upgrade_flutter_script:
+    # Master uses a pinned, auto-rolled version to prevent out-of-band CI
+    # failures due to changes in Flutter.
+    # TODO(stuartmorgan): Investigate an autoroller for stable as well.
+    - TARGET_TREEISH=$CHANNEL
+    - if [[ "$CHANNEL" == "master" ]]; then
+    -   TARGET_TREEISH=$(< .ci/flutter_$CHANNEL.version)
+    - fi
     # Ensure that the repository has all the branches.
     - cd $FLUTTER_HOME
     - git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
     - git fetch origin
-    # Switch to the requested branch.
-    - git checkout $CHANNEL
-    # Reset to upstream branch, rather than using pull, since the base image
-    # can sometimes be in a state where it has diverged from upstream (!).
-    - git reset --hard @{u}
+    # Switch to the requested channel.
+    - git checkout $TARGET_TREEISH
+    # When using a branch rather than a hash, reset to the upstream branch
+    # rather than using pull, since the base image can sometimes be in a state
+    # where it has diverged from upstream (!).
+    - if [[ "$TARGET_TREEISH" == "$CHANNEL" ]]; then
+    -   git reset --hard @{u}
+    - fi
     # Run doctor to allow auditing of what version of Flutter the run is using.
     - flutter doctor -v
   << : *TOOL_SETUP_TEMPLATE