internal/relui: fix a flake

We can't rely on the sha256 for an artifact being there at the same time
as the artifact.

Fixes golang/go#53361.

Change-Id: I7827c9e20ba0ebcdb765b6a29e35998e36131eec
Reviewed-on: https://go-review.googlesource.com/c/build/+/411903
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
diff --git a/internal/relui/workflows.go b/internal/relui/workflows.go
index c3a7098..c1eb400 100644
--- a/internal/relui/workflows.go
+++ b/internal/relui/workflows.go
@@ -697,7 +697,7 @@
 	if modifiedBySigning {
 		hash, err := fs.ReadFile(stagingFS, version+"/signed/"+a.Filename+".sha256")
 		if err != nil {
-			return artifact{}, false, err
+			return artifact{}, false, nil
 		}
 		signed.Size = int(fi.Size())
 		signed.SHA256 = string(hash)