Build bazel release binaries with -c opt.

This saves ~5MB on linux (probably the same on other platforms as well).

Progress on #6314.

RELNOTES: None

.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"