Update runtests.ps1

Author: HaoK

eng/helix/content/runtests.ps1

@@ -14,6 +14,7 @@ param(
 
 $env:DOTNET_SKIP_FIRST_TIME_EXPERIENCE = 1
 $env:DOTNET_MULTILEVEL_LOOKUP = 0
+$env:InstallPlaywright = "$InstallPlaywright"
 $env:PLAYWRIGHT_BROWSERS_PATH = "$currentDirectory\ms-playwright"
 $env:PLAYWRIGHT_DRIVER_PATH = "$currentDirectory\.playwright\win-x64\native\playwright.cmd"