I am getting a weird test failure while trying to build latest git nix in nixpkgs (x86_64-linux)
https://github.com/NixOS/nixpkgs/pull/370305

../src/libexpr-tests/nix_api_expr.cc:138: Failure
Expected equality of these values:
  true
  is_valid_path
    Which is: false

[  FAILED  ] nix_api_expr_test.nix_build_drv (50 ms)
unknown file: Failure
C++ exception with description "nix_err_code(ctx) != NIX_OK, message: error: builder for '/build/tests_nix-store.ma223o/my_nix_store/c9y864ic046rwpb15hlgwrz8n251rb0r-letsbuild.drv' failed with exit code 1" thrown in the test body.

[  FAILED  ] nix_api_expr_test.nix_expr_realise_context (58 ms)

how to fix it?

Yeah, I'm aware of running via an alternative physical location for a store. I mean to actually change the prefix against which things are built. (yep, will invalidate the cache)
This does seem to be sort-of-working:

[tom@tframe:~/nix/main/build]$ ./src/nix/nix build --impure --expr 'derivation {name="a"; builder = ./busybox; system="x86_64-linux"; args = ["sh" "-c" "echo hi > $out"];}' -vv -L --builders ''
evaluating file '<nix/derivation-internal.nix>'
downloading 'https://cache.nixos.org/nix-cache-info'...
warning: binary cache 'https://cache.nixos.org' is for Nix stores with prefix '/nix/store', not '/opt/nix/store'
this derivation will be built:
  /opt/nix/store/77iwbxab9l1fsng3vlax7wf7vi5diz6v-a.drv
building '/opt/nix/store/77iwbxab9l1fsng3vlax7wf7vi5diz6v-a.drv'...