Update bazel-release.yml (#1767)

Fixes https://github.com/buildkite/agent/issues/2446
diff --git a/pipelines/bazel-release.yml b/pipelines/bazel-release.yml
index c0217a9..e2dc2e1 100644
--- a/pipelines/bazel-release.yml
+++ b/pipelines/bazel-release.yml
@@ -229,7 +229,7 @@
     agents:
       - queue=windows
     command: |
-      set BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%
+      set "BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%"
 
       git fetch origin ${BUILDKITE_BRANCH}
       git checkout ${BUILDKITE_BRANCH}
@@ -260,7 +260,7 @@
       # current we just use x86_64 machines to do cross compile.
       - queue=windows
     command: |
-      set BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%
+      set "BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%"
 
       git fetch origin ${BUILDKITE_BRANCH}
       git checkout ${BUILDKITE_BRANCH}
@@ -397,7 +397,7 @@
     agents:
       - queue=windows
     command: |
-      set BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%
+      set "BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%"
 
       git fetch origin ${BUILDKITE_BRANCH}
       git checkout ${BUILDKITE_BRANCH}
@@ -418,7 +418,7 @@
       # current we just use x86_64 machines to do cross compile.
       - queue=windows
     command: |
-      set BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%
+      set "BUILDKITE_MESSAGE=%BUILDKITE_MESSAGE:~0,1000%"
 
       git fetch origin ${BUILDKITE_BRANCH}
       git checkout ${BUILDKITE_BRANCH}