Cygwin: Drop 'make check' from AppVeyor

Appveyor has quite restrictive concurrent job limits for free projects,
so reduce the job's runtime by dropping 'make check' to avoid blocking
(since we're now also running that in a GitHub action).

.appveyor.yml

@@ -66,8 +66,7 @@ build_script: >-
     make &&
     make install &&
     (cd */newlib; make info man) &&
-    (cd */newlib; make install-info install-man) &&
+    (cd */newlib; make install-info install-man)"
-    (cd */winsup; test "${BUILD}" != 'x86_64-pc-cygwin' || make check || true)"
 
 test: off
 deploy: off