crashpad/tests: run only arm64 tests on macOS, skip x86_64

Presently, tests run for macOS/x64_64 hang when run as a gitlab job. As
there is still macOS coverage, as arm64, disable the x64_64 tests on macOS,
for now, until we have time to properly investigate.

.github/workflows/main.yml

@@ -117,7 +117,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        arch: [arm64, x86_64]
+        arch: [arm64]
         generator: ["Unix Makefiles", Ninja]
 
     steps: