Revert "We only have desktop alphas based on > 68 ESR for now"

This reverts commit a53e12f3.

Makefile

@@ -42,7 +42,7 @@ release-src: submodule-update
 	$(rbm) build release --target release --target torbrowser-src
 
 alpha: submodule-update
-	$(rbm) build release --target alpha --target torbrowser-all-desktop
+	$(rbm) build release --target alpha --target torbrowser-all
 
 alpha-android: submodule-update
 	$(rbm) build release --target alpha --target torbrowser-all-android