Move PyTorch-from-source docker builds to linux.12xlarge

The gcc11 and zephyr-sdk docker-build jobs on linux.4xlarge OOM mid
PyTorch source build (~step 5000/7393 of mkl-dnn), manifesting as
"self-hosted runner lost communication with the server". Bumping to
linux.12xlarge (96 GB) for just these two images keeps the other
x86_64 from-source images on 4xlarge for now.

Authored with Claude.

Author: mergennachin

.github/workflows/docker-builds.yml

@@ -33,17 +33,20 @@ jobs:
       matrix:
         runner: [linux.4xlarge]
         docker-image-name: [
-          executorch-ubuntu-22.04-gcc11,
           executorch-ubuntu-22.04-gcc9-nopytorch,
           executorch-ubuntu-22.04-clang12,
           executorch-ubuntu-22.04-linter,
           executorch-ubuntu-22.04-arm-sdk,
-          executorch-ubuntu-22.04-zephyr-sdk,
           executorch-ubuntu-22.04-qnn-sdk,
           executorch-ubuntu-22.04-mediatek-sdk,
           executorch-ubuntu-22.04-clang12-android
         ]
         include:
+          # PyTorch is built from source in these images; 4xlarge OOMs mid-build.
+          - docker-image-name: executorch-ubuntu-22.04-gcc11
+            runner: linux.12xlarge
+          - docker-image-name: executorch-ubuntu-22.04-zephyr-sdk
+            runner: linux.12xlarge
           - docker-image-name: executorch-ubuntu-22.04-gcc11-aarch64
             runner: linux.arm64.2xlarge
           - docker-image-name: executorch-ubuntu-22.04-gcc11-aarch64-android