npmHooks.npmInstallHook: allow disabling npm prune invocation

In some odd scenarios, `npm prune` either fails, or hangs. I have no idea
what could possibly be wrong at the moment, but let's provide an escape
hatch for packages that can still use the rest of the install hook's
functionality.

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

@@ -27,7 +27,10 @@ npmInstallHook() {
     local -r nodeModulesPath="$packageOut/node_modules"
 
     if [ ! -d "$nodeModulesPath" ]; then
+        if [ -z "${dontNpmPrune-}" ]; then
             npm prune --omit dev --no-save $npmInstallFlags "${npmInstallFlagsArray[@]}" $npmFlags "${npmFlagsArray[@]}"
+        fi
+
         find node_modules -maxdepth 1 -type d -empty -delete
 
         cp -r node_modules "$nodeModulesPath"