Build bazel release binaries with -c opt.

This saves ~5MB on every platform, new sizes:
104MB on Linux
107MB on Mac
98MB on Windows

#7492 will probably land before this and reduce size by another ~10MB, so we should be below 100MB on every platform!

Progress on #6314.

RELNOTES: None

Closes #7494.

PiperOrigin-RevId: 235010390

.bazelci/build_bazel_binaries.yml

@@ -7,16 +7,24 @@ platforms:
     - rm -f WORKSPACE.bak
     build_targets:
     - "//src:bazel"
+    build_flags:
+    - "-c"
+    - "opt"
   macos:
     shell_commands:
     - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
       android_ndk_repository/android_ndk_repository/' WORKSPACE
     - rm -f WORKSPACE.bak
     build_targets:
     - "//src:bazel"
+    build_flags:
+    - "-c"
+    - "opt"
   windows:
     build_flags:
     - "--copt=-w"
     - "--host_copt=-w"
+    - "-c"
+    - "opt"
     build_targets:
     - "//src:bazel"