vg: move NIX_CFLAGS_COMPILE into env for consistency

The rest of nixpkgs (minus some stragglers) has had this moved into env.

This commit does actually cause a rebuild since structuredAttrs is already enabled.

pkgs/by-name/vg/vg/package.nix

@@ -128,14 +128,15 @@ stdenv.mkDerivation (finalAttrs: {
     ]
   );
 
-  # needed, else build fails
-  env.VG_GIT_VERSION = finalAttrs.src.tag;
-
-  # deps/elfutils
-  NIX_CFLAGS_COMPILE = toString [
-    "-Wno-error=stringop-overflow"
-    "-Wno-error=unterminated-string-initialization"
-  ];
+  env = {
+    # needed, else build fails
+    VG_GIT_VERSION = finalAttrs.src.tag;
+    # deps/elfutils
+    NIX_CFLAGS_COMPILE = toString [
+      "-Wno-error=stringop-overflow"
+      "-Wno-error=unterminated-string-initialization"
+    ];
+  };
 
   makeFlags = [
     # don't build statically