chore(3p/nix): apply google-readability-casting

Command run: jq <compile_commands.json -r 'map(.file)|.[]' | grep -v '/generated/' | parallel clang-tidy -p compile_commands.json -checks=-*,google-readability-casting --fix

Manual fixes applied in src/nix-env/nix-env.cc, src/libstore/store-api.cc

Change-Id: I406b4be9368c557ca59329bf6f7002704e955f8d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1557
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/nix-channel/nix-channel.cc

@@ -113,7 +113,7 @@ static void update(const StringSet& channelNames) {
     std::smatch match;
     auto urlBase = baseNameOf(url);
     if (std::regex_search(urlBase, match, std::regex("(-\\d.*)$"))) {
-      cname = cname + (std::string)match[1];
+      cname = cname + std::string(match[1]);
     }
 
     std::string extraAttrs;