Add multi-line support to pre/post_install (fixes #1890) (#1980)

* Add multi-line support to pre/post_install (fixes #1890)

* iex (@($p.._install) -join "`r`n")

Author: rasa

lib/install.ps1

@@ -884,15 +884,15 @@ function pre_install($manifest, $arch) {
     $pre_install = arch_specific 'pre_install' $manifest $arch
     if($pre_install) {
         write-output "Running pre-install script..."
-        iex $pre_install
+        iex (@($pre_install) -join "`r`n")
     }
 }
 
 function post_install($manifest, $arch) {
     $post_install = arch_specific 'post_install' $manifest $arch
     if($post_install) {
         write-output "Running post-install script..."
-        iex $post_install
+        iex (@($post_install) -join "`r`n")
     }
 }