Remove support for ARM64 platforms in build workflow

We cannot currently test _any_ ARM64 platforms under qemu because qemu
cannot currently support iptables.  See multiarch/qemu-user-static#191
for more details.

.github/workflows/build.yml

@@ -179,7 +179,10 @@ jobs:
       matrix:
         architecture:
           - amd64
-          - arm64
+          # We cannot currently test _any_ ARM64 platforms under qemu
+          # because qemu cannot currently support iptables:
+          # https://github.com/multiarch/qemu-user-static/issues/191
+          # - arm64
         platform:
           # Amazon Linux 2 does not appear to provide OpenVPN
           # - amazonlinux2023-systemd

README.md

@@ -39,6 +39,11 @@ ufw disable && ufw enable
 These steps can be performed via cloud-init, as is done
 [here](https://github.com/cisagov/openvpn-server-tf-module/blob/develop/cloudinit/create-iptables-rule-for-nat.sh).
 
+> [!NOTE]
+> We cannot currently test *any* ARM64 platforms under `qemu` because
+> [`qemu` cannot currently support
+> `iptables`](https://github.com/multiarch/qemu-user-static/issues/191).
+
 ## Requirements ##
 
 None.