Make build.ps1 executable

Make build.ps1 executable (chmod +x) and execute using `/usr/bin/env`
so the `PATH` is taken into consideration when finding the `pwsh`
binary.

Author: asbjornu

build.ps1

@@ -1,4 +1,4 @@
-#!/usr/bin/pwsh
+#!/usr/bin/env pwsh
 <#
 .PARAMETER Stage
 The build stage to execute.