ci: build musl only for arm64 and reuse toolchain cache [CI]

packaging/container/Agent.NativeBuild.Containerfile

@@ -4,7 +4,7 @@ FROM --platform=linux/amd64 ${NATIVEAOT_BUILDER_IMAGE} AS toolchain
 FROM toolchain AS restore
 WORKDIR /src/backend
 COPY restore/backend/ ./
-ARG NODE_RID=linux-musl-x64
+ARG NODE_RID=linux-musl-arm64
 ARG STATIC_MSQUIC=true
 ENV NUGET_PACKAGES=/root/.nuget/packages
 # Layer hits skip restore. A dependency change still reuses downloaded packages.