gzip: make reproducible when GZIP_NO_TIMESTAMPS is set

the logic can be tested with

```
$ GZIP_NO_TIMESTAMPS=1 && echo "${GZIP_NO_TIMESTAMPS:+-n }"
-n
$ unset GZIP_NO_TIMESTAMPS && echo "${GZIP_NO_TIMESTAMPS:+-n }"

```

pkgs/stdenv/generic/setup.sh

@@ -1348,6 +1348,10 @@ showPhaseFooter() {
 
 
 genericBuild() {
+    # variable used by our gzip wrapper to add -n.
+    # gzip is in common-path.nix and is added to nix-shell but we only want to change its behaviour in nix builds. do not move to a setupHook in gzip.
+    export GZIP_NO_TIMESTAMPS=1
+
     if [ -f "${buildCommandPath:-}" ]; then
         source "$buildCommandPath"
         return

pkgs/tools/compression/gzip/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv
 , fetchurl
+, makeWrapper
 , xz
-, writeText
 }:
 
 # Note: this package is used for bootstrapping fetchurl, and thus
@@ -22,7 +22,7 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
-  nativeBuildInputs = [ xz.bin ];
+  nativeBuildInputs = [ xz.bin makeWrapper ];
 
   makeFlags = [
     "SHELL=/bin/sh"
@@ -40,12 +40,12 @@ stdenv.mkDerivation rec {
   preFixup = ''
     sed -i '1{;/#!\/bin\/sh/aPATH="'$out'/bin:$PATH"
     }' $out/bin/*
-  '';
-
-  # set GZIP env variable to "-n" to stop gzip from adding timestamps
+  ''
+  # run gzip with "-n" when $GZIP_NO_TIMESTAMPS (set by stdenv's setup.sh) is set to stop gzip from adding timestamps
   # to archive headers: https://github.com/NixOS/nixpkgs/issues/86348
-  setupHook = writeText "setup-hook" ''
-    export GZIP="-n"
+  + ''
+    wrapProgram $out/bin/gzip \
+      --add-flags "\''${GZIP_NO_TIMESTAMPS:+-n}"
   '';
 
   meta = {