fix(glue): Set BUILD_TOP correctly as per nix behavior.

This change makes BUILD_TOP to point to /build, which is what nix does.

Change-Id: I4ffef67aff0665d13859378a86329291a53d4ea0
Reviewed-on: https://cl.snix.dev/c/snix/+/30500
Reviewed-by: Florian Klink <flokli@flokli.de>
Autosubmit: Vova Kryachko <v.kryachko@gmail.com>
Tested-by: besadii

snix/glue/src/snix_build.rs

@@ -21,7 +21,7 @@ use crate::known_paths::KnownPaths;
 const NIX_ENVIRONMENT_VARS: [(&str, &str); 12] = [
     ("HOME", "/homeless-shelter"),
     ("NIX_BUILD_CORES", "0"), // TODO: make this configurable?
-    ("NIX_BUILD_TOP", "/"),
+    ("NIX_BUILD_TOP", "/build"),
     ("NIX_LOG_FD", "2"),
     ("NIX_STORE", "/nix/store"),
     ("PATH", "/path-not-set"),