stdenv: fix env pass through with structured attrs

When using structured attributes, `env` attributes must not be
derivation arguments.

Fixes the following example:
```
let
  drv = stdenv.mkDerivation {
    name = "foo";
    __structuredAttrs = true;
    env.FOO = "foo";
  };
in
stdenv.mkDerivation drv.drvAttrs
```

Note that passthru takes precedence over env.

```
nix-repl> (stdenv.mkDerivation {
            name = "foo";
            env.FOO = "foo";
            passthru.FOO = "bar";
          }).FOO
"bar"
```

pkgs/stdenv/generic/make-derivation.nix

@@ -641,8 +641,14 @@ extendDerivation
    # Pass through extra attributes that are not inputs, but
    # should be made available to Nix expressions using the
    # derivation (e.g., in assertions).
+   #
+   # For structured attributes, we do not want checkedEnv to end up in
+   # derivation arguments (i.e. drvAttrs) to allow reusing them as mkDerivation
+   # argument again (checkedEnv does not allow collisions between derivation
+   # arguments and `env` contents).
+   optionalAttrs (envIsExportable && __structuredAttrs) checkedEnv //
    passthru)
-  (derivation (derivationArg // optionalAttrs envIsExportable checkedEnv));
+  (derivation (derivationArg // optionalAttrs (envIsExportable && !__structuredAttrs) checkedEnv));
 
 in
 {