Only build for x86_64

Mindustry does not support arm[0].
This enables only the build for x86_64[1].

Fixes flathub#66.

[0]: Anuken/Arc#167
[1]: https://docs.flathub.org/docs/for-app-authors/maintenance#limiting-the-set-of-architectures-to-build-on

flathub.json

@@ -1,3 +1,4 @@
 {
-  "automerge-flathubbot-prs": true
+  "automerge-flathubbot-prs": true,
+  "only-arches": ["x86_64"]
 }