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/binary-cache-store.cc

@@ -30,7 +30,7 @@ BinaryCacheStore::BinaryCacheStore(const Params& params) : Store(params) {
   }
 
   StringSink sink;
-  sink << narVersionMagic1;
+  sink << std::string(kNarVersionMagic1);
   narMagic = *sink.s;
 }