ci: Set up emulation and build for arm64

Previous commits enable this to work.

Emulation is necessary to build arm64 on an amd64 host machine¹.

¹ https://github.com/docker/build-push-action/blob/965c6a410d446a30e95d35052c67d6eded60dad6/docs/advanced/multi-platform.md

.github/workflows/ci.yml

@@ -30,6 +30,8 @@ jobs:
       # and hyphens.
       run: echo "TAG=branch-${GITHUB_REF_NAME//[^A-Za-z0-9._-]/-}" | tee -a $GITHUB_ENV
 
+    - uses: docker/setup-qemu-action@v2
+
     # GITHUB_TOKEN is unreliable¹ so use a token from nextstrain-bot.
     # ¹ https://github.com/docker/build-push-action/issues/463#issuecomment-939394233
     - uses: docker/login-action@v1
@@ -38,7 +40,7 @@ jobs:
         username: nextstrain-bot
         password: ${{ secrets.GH_TOKEN_NEXTSTRAIN_BOT_MANAGE_PACKAGES }}
 
-    - run: ./devel/build -p linux/amd64 -r ghcr.io -t $TAG
+    - run: ./devel/build -p linux/amd64,linux/arm64 -r ghcr.io -t $TAG
 
     outputs:
       tag: ${{ env.TAG }}