Specify --compilation_mode=opt for gVisor runsc releases.

This turns on optimizations for release builds.

Detected in [this comment](freedomofpress/dangerzone#590 (comment)).

PiperOrigin-RevId: 642076108

Author: EtiennePerot

.buildkite/release.yaml

@@ -37,8 +37,8 @@ steps:
     label: ":ship: Release"
     if: build.branch == "master" || build.tag != null
     commands:
-      - make BAZEL_OPTIONS=--config=x86_64 artifacts/x86_64
-      - make BAZEL_OPTIONS=--config=aarch64 artifacts/aarch64
+      - "make BAZEL_OPTIONS='--config=x86_64 --compilation_mode=opt' artifacts/x86_64"
+      - "make BAZEL_OPTIONS='--config=aarch64 --compilation_mode=opt' artifacts/aarch64"
       - make release RELEASE_NIGHTLY=$$RELEASE_NIGHTLY
       - cd repo && gsutil cp -r . gs://gvisor/releases/
   - <<: *common