local: mark repository files as read-only

This is intended to prevent accidental modifications of data files.
Marking the files as read-only was accidentally removed in #1258.

internal/backend/local/local_unix.go

@@ -9,6 +9,6 @@ import (
 )
 
 // set file to readonly
-func setNewFileMode(f string, mode os.FileMode) error {
-	return fs.Chmod(f, mode)
+func setFileReadonly(f string, mode os.FileMode) error {
+	return fs.Chmod(f, mode&^0222)
 }