VS2017 image: Error cloning repository

[frankmorgner]

Some of our builds are aborting before the actual build steps are executed. We're running 4 different configurations on the Visual Studio 2017 image and in 50% of the time, there is one build which is aborted after 5 seconds with the following (complete!) log:

Build started
git clone -q --branch=master https://github.com/OpenSC/OpenSC.git C:\projects\opensc
      1 [main] git 1483 dofork: child -1 - forked process 3100 died unexpectedly, retry 0, exit code 0xC0000142, errno 11
error: cannot fork() for git-remote-https: Resource temporarily unavailable
Command exited with code 255

This problem does not seem to happen when the Visual Studio 2015 image is used. It seems that the problem is not related to our specific build steps, since those are not yet executed.

The complete AppVeyor configuration can be found here:
https://github.com/OpenSC/OpenSC/blob/master/.appveyor.yml
An example build is available here:
https://ci.appveyor.com/project/frankmorgner/opensc/builds/49237045/job/749q0b18thda95be