Test x86_64-apple-darwin on newer CI machines (bytecodealliance#9880)

This commit moves testing of x86_64-apple-darwin from `macos-13` macs
to the `macos-14` image which is newer. The hope is that these newer
images, being natively M1 machines, are faster to compile and less
flaky. A number of CI builds recently have failed due to seeming CI
issues with the older machines, so this'll hopefully keep the same
amount of testing while improving robustness.

Author: alexcrichton

ci/build-test-matrix.js

@@ -81,9 +81,10 @@ const FULL_MATRIX = [
     "isa": "x64"
   },
   {
-    "os": "macos-13",
+    "os": macos,
     "name": "Test macOS x86_64",
     "filter": "macos-x64",
+    "target": "x86_64-apple-darwin",
   },
   {
     "os": macos,