treewide: replace fmt.Errorf with fmt.Sprintf in panic calls

Since we are inside a panic anyway, let's not use Errorf as we only need
a string.

Change-Id: Id43e403124e0fa60e038397d3153f96181c916e2
Reviewed-on: https://review.monogon.dev/c/monogon/+/3034
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/cloud/agent/main.go b/cloud/agent/main.go
index 16d5b17..45c3e52 100644
--- a/cloud/agent/main.go
+++ b/cloud/agent/main.go
@@ -39,7 +39,7 @@
 		}
 		reader, err := lt.Read("", logtree.WithChildren(), logtree.WithStream())
 		if err != nil {
-			panic(fmt.Errorf("could not set up root log reader: %v", err))
+			panic(fmt.Sprintf("could not set up root log reader: %v", err))
 		}
 		go func() {
 			for {