cloudbuild: cross-compile to ARM no longer works

The BUILD files for Rust dependencies that are generated by
`cargo raze` are specific to a particular configured `target`
architecture triple, in our case "x86_64-unknown-linux-gnu".

This means that cross-compiles don't work, in particular prost
and proc-macro fail to build.

cloudbuild.yaml

@@ -46,12 +46,6 @@ steps:
     timeout: 60m
     entrypoint: 'bash'
     args: ['./scripts/build_server', '-s', 'base']
-  - name: 'gcr.io/oak-ci/oak:latest'
-    id: build_server_arm
-    waitFor: ['bazel_init']
-    timeout: 60m
-    entrypoint: 'bash'
-    args: ['./scripts/build_server', '-s', 'arm']
 
   # Package the Hello World application in a Docker image.
   - name: 'gcr.io/oak-ci/oak:latest'