Skip coverage checks for cross-builds

Sometimes it's not consistent across builds for no apparent reason
https://github.com/rspec/rspec-support/runs/1626127309
rspec/rspec-support#477 (comment)

Author: pirj

ci/script/run_build

@@ -27,6 +27,7 @@ fi
 
 if supports_cross_build_checks; then
   fold "one-by-one specs" run_specs_one_by_one
+  export NO_COVERAGE=true
   run_all_spec_suites
 else
   echo "Skipping the rest of the build on non-MRI rubies"