release: upload to Google Play with fastlane

Replace the unpinned third-party upload-google-play action with fastlane
supply from a small nix release devshell, and pin the remaining
third-party GitHub release action to a commit SHA.

flake.nix

@@ -83,6 +83,10 @@
 
             inherit shellHook;
           };
+
+          release = pkgs.mkShell {
+            packages = [ pkgs.fastlane ];
+          };
         }
       );
     };