refactor(3p/nix): Apply clang-tidy's modernize-* fixes

This applies the modernization fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

The 'modernize-use-trailing-return-type' fix was excluded due to my
personal preference (more specifically, I think the 'auto' keyword is
misleading in that position).

third_party/nix/src/libexpr/attr-path.cc

@@ -15,7 +15,7 @@ static Strings parseAttrPath(const string& s) {
       cur.clear();
     } else if (*i == '"') {
       ++i;
-      while (1) {
+      while (true) {
         if (i == s.end()) {
           throw Error(format("missing closing quote in selection path '%1%'") %
                       s);