fix: set fileSize to full length instead of unread portion (#18409)

`content.Len()` would return `0` bytes after a file was fully read.
Since the buffer `Len` function returns the length of the unread
portion.

coderd/files/cache.go

@@ -35,7 +35,7 @@ func NewFromStore(store database.Store, registerer prometheus.Registerer, authz
 		return CacheEntryValue{
 			Object: file.RBACObject(),
 			FS:     archivefs.FromTarReader(content),
-			Size:   int64(content.Len()),
+			Size:   int64(len(file.Data)),
 		}, nil
 	}