Remove riscv64 from build platforms (#633)

The docker image `ubuntu:22.04` is not available for `riscv64`, which limits options for providing support for it here.

Author: das7pad

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
               DOCKER_IMAGE=phusion/baseimage
               GIT_BRANCH=${GITHUB_REF##*/}
               # Set the platforms to build for here and thus reduce duplicating it.
-              PLATFORMS=amd64,arm,arm64,riscv64
+              PLATFORMS=amd64,arm,arm64
               TAGS="${DOCKER_IMAGE}:${GIT_BRANCH}, ghcr.io/${{ github.repository_owner }}/baseimage:${GIT_BRANCH}"
               
               # Set output parameters.