enable linux/riscv64 builds

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

.github/workflows/build.yml

@@ -14,7 +14,7 @@ on:
 env:
   REPO_SLUG_ORIGIN: "moby/buildkit:master"
   CACHEKEY_BINARIES: "binaries"
-  PLATFORMS: "linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le"
+  PLATFORMS: "linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le,linux/riscv64"
 
 jobs:
   base:

hack/cross

@@ -3,7 +3,7 @@
 . $(dirname $0)/util
 set -e
 
-: ${TARGETPLATFORM=linux/amd64,linux/arm/v7,linux/arm64,darwin/amd64,windows/amd64,linux/ppc64le,linux/s390x}
+: ${TARGETPLATFORM=linux/amd64,linux/arm/v7,linux/arm64,darwin/amd64,windows/amd64,linux/ppc64le,linux/s390x,linux/riscv64}
 : ${EXPORT_LOCAL=}
 
 importCacheFlags=""