explicitly use set-variable to work around a bug some users saw where the command invocation would vail

provisioner/powershell/provisioner.go

@@ -147,11 +147,11 @@ func (p *Provisioner) Prepare(raws ...interface{}) error {
 	}
 
 	if p.config.ExecuteCommand == "" {
-		p.config.ExecuteCommand = `powershell -executionpolicy bypass "& { if (Test-Path variable:global:ProgressPreference){$ProgressPreference='SilentlyContinue'};. {{.Vars}}; &'{{.Path}}';exit $LastExitCode }"`
+		p.config.ExecuteCommand = `powershell -executionpolicy bypass "& { if (Test-Path variable:global:ProgressPreference){set-variable -name variable:global:ProgressPreference -value 'SilentlyContinue'};. {{.Vars}}; &'{{.Path}}';exit $LastExitCode }"`
 	}
 
 	if p.config.ElevatedExecuteCommand == "" {
-		p.config.ElevatedExecuteCommand = `powershell -executionpolicy bypass "& { if (Test-Path variable:global:ProgressPreference){$ProgressPreference='SilentlyContinue'};. {{.Vars}}; &'{{.Path}}'; exit $LastExitCode }"`
+		p.config.ElevatedExecuteCommand = `powershell -executionpolicy bypass "& { if (Test-Path variable:global:ProgressPreference){set-variable -name variable:global:ProgressPreference -value 'SilentlyContinue'};. {{.Vars}}; &'{{.Path}}';exit $LastExitCode }"`
 	}
 
 	if p.config.Inline != nil && len(p.config.Inline) == 0 {