[build] use TARGETPLATFORM instead

.github/actions/muslbuilder/Dockerfile

@@ -1,4 +1,4 @@
-FROM --platform=$BUILDPLATFORM alpine:3.16
+FROM --platform=$TARGETPLATFORM alpine:3.16
 
 LABEL com.github.actions.name="C++ MUSL Builder Slim"
 LABEL com.github.actions.description="Provides a C++ MUSL environment"