DO NOT MERGE ci: run arm 64-bit and 32-bit build test jobs on aarch64 runners

This should work around this issue, at least for pmaports (where it's hit quite frequently):

postmarketOS/postmarketos#83

The downside of this is that there is (currently) only 1 aarch64 runner,
so CI pipelines may take a bit longer to complete than before. IMHO
that's better than having jobs that straight up fail (often multiple
times) and (for some MRs) effectively losing build testing for ARM stuff
pre-merge in gitlab CI.

We'll want to revert this when the above issue is resolved, to restore
the previous/higher CI throughput.

.gitlab-ci.yml

@@ -154,11 +154,15 @@ build-x86:
     - .ci/build-x86.sh
 
 build-aarch64:
+  tags:
+    - aarch64
   extends: .build
   script:
     - .ci/build-aarch64.sh
 
 build-armv7:
+  tags:
+    - aarch64
   extends: .build
   script:
     - .ci/build-armv7.sh