refactor(tvix): Make static strings constexpr string_views

Make all static std::strings constexpr std::string_views, and replace
concatenation with absl::StrCat where necessary.

Technically all of these are constant, so they really don't need to be
top-level statics - and since I'm trying to get rid of as much global
state as possible in preparation for making the nix daemon properly
multithreaded I figured I'd knock these out while I was at it.

Change-Id: Ibd3ad9ef68f0a0eacb135541b39fdb13dae042e1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1939
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

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

@@ -531,13 +531,13 @@ unsigned int RpcStore::getProtocol() { return PROTOCOL_VERSION; }
 
 }  // namespace store
 
-static std::string uriScheme = "unix://";
+constexpr std::string_view kUriScheme = "unix://";
 
 // TODO(grfn): Make this a function that we call from main rather than... this
 static RegisterStoreImplementation regStore([](const std::string& uri,
                                                const Store::Params& params)
                                                 -> std::shared_ptr<Store> {
-  if (std::string(uri, 0, uriScheme.size()) != uriScheme) {
+  if (std::string(uri, 0, kUriScheme.size()) != kUriScheme) {
     return nullptr;
   }
   auto channel = grpc::CreateChannel(uri, grpc::InsecureChannelCredentials());