feat: stop cross compiling miniooni (#908)

See https://github.com/ooni/probe/issues/2119

While there, shrink the Makefile to only export the minimum
set of rules to comfortable build using GHA.

MOBILE/gomobile

@@ -42,12 +42,15 @@ else
 	OONI_PSIPHON_TAGS=""
 fi
 
-MAYBE_ANDROID_API=""
 if [[ $TARGET == "android" ]]; then
 	ANDROID_HOME=$(./MOBILE/android/home)
 	NDK_VERSION=$(cat NDKVERSION)
 	ANDROID_NDK_HOME=$ANDROID_HOME/ndk/$NDK_VERSION
 	MAYBE_ANDROID_API="-androidapi 21"
+else
+	ANDROID_HOME=""
+	ANDROID_NDK_HOME=""
+	MAYBE_ANDROID_API=""
 fi
 
 PRODUCT=$(basename $PACKAGE)