Refactor validate for GitHub Actions

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

hack/validate-vendor

@@ -1,40 +1,20 @@
 #!/usr/bin/env sh
-
 set -eu
 
-: ${CONTINUOUS_INTEGRATION=}
-: ${DOCKER_BUILDKIT=}
-
-progressFlag=""
-if [ "$CONTINUOUS_INTEGRATION" = "true" ]; then progressFlag="--progress=plain"; fi
-
 case ${1:-} in
-'')
-  . $(dirname $0)/util
-  case $buildmode in
-  "buildkit")
-    buildctl build $progressFlag --frontend=dockerfile.v0 --local context=. --local dockerfile=. --frontend-opt filename=./hack/dockerfiles/vendor.Dockerfile --frontend-opt target=validate
+  '')
+    . $(dirname $0)/util
+    buildxCmd build \
+      --target validate \
+      --file ./hack/dockerfiles/vendor.Dockerfile \
+      .
     ;;
-  "docker-buildkit")
-    export DOCKER_BUILDKIT=1
-    iidfile=$(mktemp -t docker-iidfile.XXXXXXXXXX)
-    docker build --iidfile $iidfile -f ./hack/dockerfiles/vendor.Dockerfile --target validate --force-rm . || exit 1
-    iid=$(cat $iidfile)
-    docker rmi $iid
-    rm -f $iidfile
-    ;;
-  *)
-		echo "buildkit support is required"
-    exit 1
-    ;;
-  esac
-  ;;
-check)
-  status="$(git status --porcelain -- go.mod go.sum vendor 2>/dev/null)"
-  diffs=$(echo "$status" | grep -v '^[RAD] ' || true)
-  if [ "$diffs" ]; then
+  check)
+    status="$(git status --porcelain -- go.mod go.sum vendor 2>/dev/null)"
+    diffs=$(echo "$status" | grep -v '^[RAD] ' || true)
+    if [ "$diffs" ]; then
       {
-	set +x
+        set +x
         echo 'The result of "make vendor" differs'
         echo
         echo "$diffs"
@@ -43,7 +23,7 @@ check)
         echo
       } >&2
       exit 1
-  fi
-  echo 'Congratulations! All vendoring changes are done the right way.'
-  ;;
+    fi
+    echo 'Congratulations! All vendoring changes are done the right way.'
+    ;;
 esac