Revert "ci: Disable AArch64 Linux SDK build"

This reverts commit a1f3887 because the
new zephyr-runner v2 with the self-hosted AArch64 runners are no longer
subject to termination by other workloads.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/ci.yml

@@ -187,7 +187,7 @@ jobs:
 
         if [ "${build_host_all}" == "y" ]; then
           build_host_linux_x86_64="y"
-          # build_host_linux_aarch64="y"
+          build_host_linux_aarch64="y"
           build_host_macos_x86_64="y"
           build_host_macos_aarch64="y"
           build_host_windows_x86_64="y"