test(tvix/store): use tokio-retry for exp backoff

Rather than using this loop, use exponential backoff while waiting for
the socket path to be created.

Change-Id: I3056b1525784cd712b1d81f84876c9ca0be10dc6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9569
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -8634,6 +8634,10 @@ rec {
             name = "test-case";
             packageId = "test-case";
           }
+          {
+            name = "tokio-retry";
+            packageId = "tokio-retry";
+          }
         ];
         features = {
           "default" = [ "fuse" "tonic-reflection" ];