ci: Add linux-aarch64 test environment

This commit adds the test environment for the AArch64 Linux toolchains
and host tools using the ARM64 zephyr-runner.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/ci.yml

@@ -293,6 +293,16 @@ jobs:
             },'
         fi
 
+        if [ "${build_host_linux_aarch64}" == "y" ]; then
+          MATRIX_TESTENVS+='{
+              "name": "ubuntu-20.04-aarch64",
+              "runner": "zephyr-runner-linux-arm64-xlarge",
+              "container": "",
+              "bundle-host": "linux-aarch64",
+              "bundle-archive": "tar.xz"
+            },'
+        fi
+
         if [ "${build_host_macos_x86_64}" == "y" ]; then
           MATRIX_TESTENVS+='{
               "name": "macos-11-x86_64",