fix(release): honor target arch for plugin images

Author: kahirokunn

hack/Dockerfile.plugin

@@ -18,10 +18,10 @@
 # When mounted at /plugin, the executable is /plugin/bin/<PLUGIN_NAME>-plugin.
 
 ARG PLUGIN_NAME
-FROM golang:1.25 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.25 AS builder
 ARG PLUGIN_NAME
-ARG TARGETOS=linux
-ARG TARGETARCH=amd64
+ARG TARGETOS
+ARG TARGETARCH
 WORKDIR /src
 
 COPY go.mod go.sum ./