[ci] Invoke-WebRequest with -MaximumRetryCount 3

x-ref:
  "retry network operations 3 times"
  cygwin/cygwin-install-action#25

Author: gstrauss

.github/workflows/ci.yml

@@ -258,7 +258,7 @@ jobs:
         shell: powershell
         run: |
           # (https://github.com/cygwin/cygwin-install-action/blob/master/action.yml)
-          Invoke-WebRequest https://cygwin.com/setup-x86_64.exe -OutFile C:\setup.exe
+          Invoke-WebRequest https://cygwin.com/setup-x86_64.exe -OutFile C:\setup.exe -MaximumRetryCount 3
           # because setup is a Windows GUI app, make it part of a pipeline to make
           # PowerShell wait for it to exit
           & C:\setup.exe -qgnO -t | Out-Default