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 5 minutes and 56 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Docker Image
passed #1379572
debian-testing

00:01:19

 
  Build And Test
passed #1379573
autotools-build-and-test

00:04:37

passed #1379574
meson-build-and-test

00:01:46

 
  External
passed continuous-integration/appveyor/branch