Makefile: ensure released versions are executable

Previously we would stream the compressed output via stdin and stdout
which meant we lost the executable bit on the input file. Use a
different set of flags to bzip2 to preserve permissions.

Makefile

@@ -50,7 +50,7 @@ bin/windows/amd64/$(EXECUTABLE).exe:
 # compressed artifacts, makes a huge difference (Go executable is ~9MB,
 # after compressing ~2MB)
 %.bz2: %
-	bzip2 -c < "$<" > "$@"
+	bzip2 --keep "$<"
 %.zip: %.exe
 	zip "$@" "$<"