Use powershell to get Windows temporary dir

Leverages Powershell script to work both in Windows and WSL.

lib/vagrant/util/platform.rb

@@ -656,10 +656,8 @@ module Vagrant
         # @return [String, Nil]
         def windows_temp
           if !@_windows_temp
-            result = Util::Subprocess.execute("cmd.exe", "/c", "echo %TEMP%")
-            if result.exit_code == 0
-              @_windows_temp = result.stdout.gsub("\"", "").strip
-            end
+            result = Vagrant::Util::PowerShell.execute_cmd("(Get-Item Env:TEMP).Value")
+            @_windows_temp = result.gsub("\"", "").strip
           end
           @_windows_temp
         end

test/unit/vagrant/util/platform_test.rb

@@ -538,8 +538,8 @@ EOF
       let(:temp_dir) { 'C:\Users\User\AppData\Local\Temp' }
 
       it "should return windows temporary directory" do
-        allow(Vagrant::Util::Subprocess).to receive(:execute).
-          with("cmd.exe", "/c", "echo %TEMP%").and_return(double(exit_code: 0, stdout: temp_dir))
+        allow(Vagrant::Util::PowerShell).to receive(:execute).
+          with("(Get-Item Env:TEMP).Value").and_return(temp_dir)
         expect(subject.windows_temp).to eql(temp_dir)
       end
     end