ci(gpu): repoint GPU probe image lookup at Dockerfile.gateway

The previous awk parsed `FROM <image> AS gateway` from the now-deleted
`Dockerfile.images`. The new `Dockerfile.gateway` uses an ARG with a
default (`ARG GATEWAY_BASE_IMAGE=nvcr.io/nvidia/distroless/cc:v4.0.4`)
and `FROM ${GATEWAY_BASE_IMAGE} AS gateway`, so the old script returns
nothing.

Parse the ARG default value directly so the GPU prerequisites check
keeps using the gateway base image as a `nvidia-smi` probe target.

Author: TaylorMutch

.github/workflows/e2e-gpu-test.yaml

@@ -58,7 +58,7 @@ jobs:
       - name: Check Docker GPU prerequisites
         run: |
           docker info --format '{{json .CDISpecDirs}}'
-          GPU_PROBE_IMAGE="$(awk '$1 == "FROM" && $3 == "AS" && $4 == "gateway" { print $2; exit }' deploy/docker/Dockerfile.images)"
+          GPU_PROBE_IMAGE="$(awk -F= '/^ARG[[:space:]]+GATEWAY_BASE_IMAGE=/ { print $2; exit }' deploy/docker/Dockerfile.gateway)"
           test -n "${GPU_PROBE_IMAGE}"
           docker run --rm --device nvidia.com/gpu=all "${GPU_PROBE_IMAGE}" nvidia-smi -L