add error when running build script in PowerShell 5

Author: Pathoschild

build/scripts/prepare-install-package.ps1

@@ -8,6 +8,15 @@
 #
 
 
+##########
+## Validate environment
+##########
+if ($PSVersionTable.PSVersion.Major -lt 7) {
+    Write-Error "This script needs PowerShell 7 or later. Run it using 'pwsh' instead." # needed for `$IsWindows`
+    exit 1
+}
+
+
 ##########
 ## Read arguments
 ##########