Avoid error when powershell isn't found in path (#37188)

Previously the REQUIRED flag made CMake fail with an error if
`powershell` wasn't found on line 193. This prevented the `else` block
on lines 197-198 from ever running.

scripts/buildsystems/vcpkg.cmake

@@ -190,7 +190,7 @@ function(z_vcpkg_set_powershell_path)
             set(Z_VCPKG_POWERSHELL_PATH "${Z_VCPKG_PWSH_PATH}" CACHE INTERNAL "The path to the PowerShell implementation to use.")
         else()
             message(DEBUG "vcpkg: Could not find PowerShell Core; falling back to PowerShell")
-            find_program(Z_VCPKG_BUILTIN_POWERSHELL_PATH powershell REQUIRED)
+            find_program(Z_VCPKG_BUILTIN_POWERSHELL_PATH powershell)
             if(Z_VCPKG_BUILTIN_POWERSHELL_PATH)
                 set(Z_VCPKG_POWERSHELL_PATH "${Z_VCPKG_BUILTIN_POWERSHELL_PATH}" CACHE INTERNAL "The path to the PowerShell implementation to use.")
             else()