github and circleci: disable macos CI.

The CI is broken with something about how we invoke ld.

Lucet is in maintence mode, none of our team members use macos anymore
for development, and so we are stuck without a reasonable way to debug
this issue. Without any users on macos, it doesn't make sense to invest
in keeping CI working.

.circleci/config.yml

@@ -36,17 +36,17 @@ jobs:
   ################################
   # macOS `make test-ci` targets #
   ################################
-  mac_ci_branch:
-    executor: macos
-    steps:
-      - mac_ci:
-          cache_target: true
+  # mac_ci_branch:
+  #  executor: macos
+  #  steps:
+  #    - mac_ci:
+  #        cache_target: true
 
-  mac_ci_main:
-    executor: macos
-    steps:
-      - mac_ci:
-          cache_target: false
+  # mac_ci_main:
+  #  executor: macos
+  #  steps:
+  #    - mac_ci:
+  #        cache_target: false
 
   ###################################################
   # Linux `make test-ci` targets on old x64 backend #