nixUnstable doesn't build with gcc10

[luc65r]

Describe the bug

nixUnstable doesn't build with gcc10.
Build output: https://termbin.com/ak42o

Steps To Reproduce

1. get this pr https://github.com/NixOS/nixpkgs/pull/89793
2. nix-build -A nixUnstable

Expected behavior

It should build

nix-env --version output
nix-env (Nix) 2.4pre20201201_5a6ddb3
Additional context

[r-burns]

Can confirm, easy to repro here by swapping in gcc10Stdenv.

diff --git a/flake.nix b/flake.nix
index 9addccd63..f6b6a8474 100644
--- a/flake.nix
+++ b/flake.nix
@@ -115,7 +115,7 @@
       # 'nix.perl-bindings' packages.
       overlay = final: prev: {

-        nix = with final; with commonDeps pkgs; (stdenv.mkDerivation {
+        nix = with final; with commonDeps pkgs; (gcc10Stdenv.mkDerivation {
           name = "nix-${version}";
           inherit version;

[matthewbauer]

https://github.com/NixOS/nix/pull/4334/commits/0c6ceaa04610413be2f0582a44ada1824f485bef / https://github.com/NixOS/nix/pull/4397 should fix this

[FRidh]

gcc 10 is now in staging-next.