Fix disabling installation of stack

Fixes #429

scripts/bootstrap/bootstrap-haskell.ps1

@@ -577,8 +577,8 @@ $Msys2Shell = ('{0}\msys2_shell.cmd' -f $MsysDir)
 # The bootstrap script is always silent, since we ask relevant questions here
 $SilentExport = 'export BOOTSTRAP_HASKELL_NONINTERACTIVE=1 ;'
 
-if ($InstallStack) {
+if (!($InstallStack)) {
-  $StackInstallExport = 'export BOOTSTRAP_HASKELL_INSTALL_STACK=1 ;'
+  $StackInstallExport = 'export BOOTSTRAP_HASKELL_INSTALL_NO_STACK=1 ;'
 }
 
 if ($InstallHLS) {

scripts/hooks/stack/ghc-install.sh

@@ -1,13 +1,16 @@
-#!/bin/bash
+#!/bin/sh
+
+# !! KEEP THIS SCRIPT POSIX COMPLIANT !!
 
 # see https://docs.haskellstack.org/en/stable/yaml_configuration/#ghc-installation-customisation-experimental
 # for documentation about hooks
 
-set -euo pipefail
+set -eu
 
 case $HOOK_GHC_TYPE in
     bindist)
-        echo "$(ghcup run --ghc "$HOOK_GHC_VERSION" --install)/ghc"
+        ghcdir=$(ghcup whereis --directory ghc "$HOOK_GHC_VERSION" || ghcup run --ghc "$HOOK_GHC_VERSION" --install) || exit 3
+        printf "%s/ghc" "${ghcdir}"
         ;;
     git)
         # TODO: should be somewhat possible