goproxy: add goproxy to AppManager and use it in CI via flake.nix

Change-Id: I489d202c34219434653cfe3afbb19937dadd204e
diff --git a/core/installer/app_repository.go b/core/installer/app_repository.go
index 509c7e9..d1c5f5c 100644
--- a/core/installer/app_repository.go
+++ b/core/installer/app_repository.go
@@ -39,6 +39,7 @@
 	// "values-tmpl/qbittorrent.cue",
 	// "values-tmpl/jellyfin.cue",
 	"values-tmpl/rpuppy.cue",
+	"values-tmpl/goproxy.cue",
 	"values-tmpl/certificate-issuer-custom.cue",
 }