CI: use aarch64 gitlab runners for ARM build testing

Maybe a nicer way to work around #3275

.gitlab-ci.yml

@@ -157,19 +157,19 @@ build-x86:
 
 build-aarch64:
   extends: .build
-  resource_group: qemu
+  tags: [arm64]
   script:
     - .ci/build-aarch64.sh
 
 build-armv7:
   extends: .build
-  resource_group: qemu
+  tags: [arm64]
   script:
     - .ci/build-armv7.sh
 
 build-armhf:
   extends: .build
-  resource_group: qemu
+  tags: [arm64]
   script:
     - .ci/build-armhf.sh