CI: stop building linux/arm/v6 docker images

Go's debian-based docker images don't support that platform.

I doubt there are many docker users on armv6. The few who might be
affected can probably download a binary, or spend ten seconds to do a
build.

We used to use the alpine-based image, but that lacks git, so the builds
required extra work. That seems like a good change we don't want to
lose.

.github/workflows/test.yml

@@ -75,7 +75,6 @@ jobs:
       DOCKER_PLATFORMS: >
         linux/386
         linux/amd64
-        linux/arm/v6
         linux/arm/v7
         linux/arm64/v8
         linux/ppc64le