stdenv: handle env gracefully

[ncfavier]

Derivations not using __structuredAttrs should not attempt to set environment variables from env.

Derivations using __structuredAttrs should fail if env is not exportable.

See #175649 (comment)

[roberth]

Could you test this in pkgs/test/stdenv/default.nix?

[Artturin]

diff --git a/pkgs/test/stdenv/default.nix b/pkgs/test/stdenv/default.nix
index b27fc25356a..2274a20ff17 100644
--- a/pkgs/test/stdenv/default.nix
+++ b/pkgs/test/stdenv/default.nix
@@ -100,6 +100,25 @@ in
 
   test-env-attrset = testEnvAttrset { name = "test-env-attrset"; stdenv' = bootStdenv; };
 
+  test-env-equals-derivation = bootStdenv.mkDerivation rec {
+    name = "test-env-equals-derivation";
+    env = bootStdenv.mkDerivation {
+      name = "foo";
+      buildCommand = ''
+        mkdir $out
+        touch $out/bar
+      '';
+    };
+
+    passAsFile = [ "buildCommand" ];
+    buildCommand = ''
+      declare -p env
+      echo ''${env[0]}
+      [[ "''${env[0]}" == "${env.out}" ]]
+      touch $out
+    '';
+  };
+
   test-prepend-append-to-var = testPrependAndAppendToVar {
     name = "test-prepend-append-to-var";
     stdenv' = bootStdenv;

[ncfavier]

Confirmed that the test fails on the previous commit.