ci: do not abort prematurely when hashing files

If an excluded directory was the prefix of a file (e.g.,
.git and .gitattributes), the returned file.SkipDir error
would instruct filepath.WalkDir to abort.

Originally discussed in https://github.com/ethereum-optimism/op-geth/pull/741

internal/build/file.go

@@ -31,11 +31,13 @@ func HashFolder(folder string, exlude []string) (map[string][32]byte, error) {
 	res := make(map[string][32]byte)
 	err := filepath.WalkDir(folder, func(path string, d os.DirEntry, _ error) error {
 		// Skip anything that's exluded or not a regular file
+		if d.IsDir() {
 			for _, skip := range exlude {
 				if strings.HasPrefix(path, filepath.FromSlash(skip)) {
 					return filepath.SkipDir
 				}
 			}
+		}
 		if !d.Type().IsRegular() {
 			return nil
 		}