ci: switch appveyor to the standard visual studio 2017 image

We were previously using the preview image but there doesn't seem to be
a reason why, as the commit that originally added it didn't replace the
standard image, but just used the preview from the start.

AppVeyor Support recommended switching to the standard image in this
support thread:

https://help.appveyor.com/discussions/problems/24057-build-job-has-failed-to-start-but-backup-cloud-is-not-configured#comment_47365011

Author: pietroalbini

appveyor.yml

@@ -1,7 +1,7 @@
 environment:
   # This is required for at least an AArch64 compiler in one image, and is also
   # going to soon be required for compiling LLVM.
-  APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017 Preview
+  APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
 
   # By default schannel checks revocation of certificates unlike some other SSL
   # backends, but we've historically had problems on CI where a revocation