buildNpmPackage: push npm_config_* options into npmHooks.npmConfigHook

these options affect npm generally, and aren't specific to our
`buildNpmPackage`. pushing these options into the `npmConfigHook` (which
is used by `buildNpmPackage`, among others) makes them available to more
packages, especially to enable cross compilation.

pkgs/build-support/node/build-npm-package/default.nix

···
       # Stripping takes way too long with the amount of files required by a typical Node.js project.
       dontStrip = args.dontStrip or true;
 
-      env = {
-        npm_config_arch = stdenv.hostPlatform.node.arch;
-        npm_config_platform = stdenv.hostPlatform.node.platform;
-      } // (args.env or { });
-
       meta = (args.meta or { }) // {
         platforms = args.meta.platforms or nodejs.meta.platforms;
       };

pkgs/build-support/node/build-npm-package/hooks/default.nix

···
 {
   lib,
   srcOnly,
+  stdenv,
   makeSetupHook,
   makeWrapper,
   nodejs,
···
     substitutions = {
       nodeSrc = srcOnly nodejs;
       nodeGyp = "${nodejs}/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js";
+      npmArch = stdenv.targetPlatform.node.arch;
+      npmPlatform = stdenv.targetPlatform.node.platform;
 
       # Specify `diff`, `jq`, and `prefetch-npm-deps` by abspath to ensure that the user's build
       # inputs do not cause us to find the wrong binaries.

pkgs/build-support/node/build-npm-package/hooks/npm-config-hook.sh

···
     export HOME="$TMPDIR"
     export npm_config_nodedir="@nodeSrc@"
     export npm_config_node_gyp="@nodeGyp@"
+    export npm_config_arch="@npmArch@"
+    export npm_config_platform="@npmPlatform@"
 
     if [ -z "${npmDeps-}" ]; then
         echo