refactor(3p/nix/libexpr): Store nix::Env values in a std::vector

This has several advantages:

* we can ensure that the vector is traced by the GC
* we don't need to unsafely allocate memory to make an Env

Note that there was previously a check about the size of the
environment, but it's unclear why this was the case (git history
yielded nothing interesting) and it seems to have no effect.

Change-Id: I4998b879a728a6fb68e1bd187c521e2304e5047e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1265
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: Kane York <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/eval.hh

@@ -3,6 +3,10 @@
 #include <map>
 #include <optional>
 #include <unordered_map>
+#include <vector>
+
+#include <gc/gc_allocator.h>
+#include <gc/gc_cpp.h>
 
 #include "libexpr/attr-set.hh"
 #include "libexpr/nixexpr.hh"
@@ -28,12 +32,16 @@ struct PrimOp {
       : fun(fun), arity(arity), name(name) {}
 };
 
-struct Env {
+struct Env : public gc {
+  Env(unsigned short size) : size(size) {
+    values = std::vector<Value*, traceable_allocator<Value*>>(size);
+  }
+
   Env* up;
   unsigned short size;           // used by ‘valueSize’
   unsigned short prevWith : 14;  // nr of levels up to next `with' environment
   enum { Plain = 0, HasWithExpr, HasWithAttrs } type : 2;
-  Value* values[0];
+  std::vector<Value*, traceable_allocator<Value*>> values;
 };
 
 Value& mkString(Value& v, const std::string& s,