refactor(3p/nix): Make all single-argument constructors explicit

Implicit constructors can be confusing, especially in a codebase that
is already as unintentionally obfuscated as this one.

https://google.github.io/styleguide/cppguide.html#Explicit_Constructors

third_party/nix/src/libstore/optimise-store.cc

@@ -27,7 +27,7 @@ static void makeWritable(const Path& path) {
 
 struct MakeReadOnly {
   Path path;
-  MakeReadOnly(const Path& path) : path(path) {}
+  explicit MakeReadOnly(const Path& path) : path(path) {}
   ~MakeReadOnly() {
     try {
       /* This will make the path read-only. */