stdenv: create env-vars file before writing data to it

This fixes the regression introduced by c47a1e7
on Darwin. The creation of the file using `install` and process
substitution does not work on Darwin, you get the following complain:
```
install: skipping file '/dev/fd/63', as it was replaced while being copied
```

Fixes #335016

(cherry picked from commit d00775c)

pkgs/stdenv/generic/setup.sh

@@ -981,7 +981,13 @@ substituteAllInPlace() {
 # the environment used for building.
 dumpVars() {
  if [ "${noDumpEnvVars:-0}" != 1 ]; then
- install -m 0600 <(export 2>/dev/null) "$NIX_BUILD_TOP/env-vars" || true
+ # On darwin, install(1) cannot be called with /dev/stdin or fd from process substitution
+ # so first we create the file and then write to it
+ # See https://github.com/NixOS/nixpkgs/issues/335016
+ {
+ install -m 0600 /dev/null "$NIX_BUILD_TOP/env-vars" &&
+ export 2>/dev/null >| "$NIX_BUILD_TOP/env-vars"
+ } || true
  fi
 }