Constrain website tests to AMD64 agents.
Constrain website tests to AMD64 agents.

The jekyll/jekyll:4.0.0 image requires running on AMD64.
Note how we have images/jekyll/Dockerfile.x86_64 file which is
platform dependent.

PiperOrigin-RevId: 453484514

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor