appveyor: Skip tests for x86

These work ok for x86_64, but not reliably for x86 (in the Appveyor
WoW64 environment), probably due to Cygwin bugs.
4 jobs for piglit-appveyor in 6 minutes and 57 seconds (queued for 10 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #720342
debian-testing

00:00:42

 
  Build And Test
passed #720343
autotools-build-and-test

00:06:15

passed #720344
meson-build-and-test

00:01:42

 
  External
passed continuous-integration/appveyor/branch