Workaround Appveyor random failures due to nuget install errors

Add and use nuget-install.cmd based on
https://github.com/appveyor/ci/blob/master/scripts/nuget-restore.cmd

Tools/nuget-install.cmd

@@ -0,0 +1,28 @@
+rem Workaround 'nuget install' not being reliable by retrying a few times
+
+@echo off
+rem initiate the retry number
+set errorCode=1
+set retryNumber=0
+set maxRetries=5
+
+:RESTORE
+nuget install %*
+
+rem problem?
+IF ERRORLEVEL %errorCode% GOTO :RETRY
+
+rem everything is fine!
+GOTO :EXIT
+
+:RETRY
+@echo Oops, nuget restore exited with code %errorCode% - let us try again!
+set /a retryNumber=%retryNumber%+1
+IF %reTryNumber% LSS %maxRetries% (GOTO :RESTORE)
+IF %retryNumber% EQU %maxRetries% (GOTO :ERR)
+
+:ERR
+@echo Sorry, we tried restoring nuget packages for %maxRetries% times and all attempts were unsuccessful!
+EXIT /B 1
+
+:EXIT

appveyor.yml

@@ -46,7 +46,7 @@ install:
 - ps: $env:VSCOMNTOOLS=(Get-Content ("env:VS" + "$env:VSVER" + "0COMNTOOLS"))
 - echo "Using Visual Studio %VSVER%.0 at %VSCOMNTOOLS%"
 - call "%VSCOMNTOOLS%\..\..\VC\vcvarsall.bat" %VCVARS_PLATFORM%
-- nuget install pcre -Verbosity detailed -Version 8.33.0.1 -OutputDirectory C:\pcre
+- Tools\nuget-install.cmd pcre -Verbosity detailed -Version 8.33.0.1 -OutputDirectory C:\pcre
 - set PCRE_ROOT=C:/pcre/pcre.8.33.0.1/build/native
 - set PATH=C:\Python%VER%%LANG_PLATFORM%;%PATH%
 - python -V