rclone: check existence of file rather than using cat (#7799)

It's not really clear why this was done in the first place, and
furthermore it means that the secrets have been getting printed to
stdout and appear on the system journal as a result.

modules/programs/rclone.nix

@@ -252,7 +252,7 @@ in
           injectSecret =
             remote:
             lib.mapAttrsToList (secret: secretFile: ''
-              if ! cat "${secretFile}"; then
+              if [[ ! -r "${secretFile}" ]]; then
                 echo "Secret \"${secretFile}\" not found"
                 cleanup
               fi