Don't publish macos-x86_64

Turns out the images changed again since the last
piece of documentation I looked at, so both macOS
matrix entries built as aarch64.

This OS/arch pair is virtually deprecated at this
point and presumably GitHub will only use aarch64
macOS runners going forwards, so let's drop the
complexity.

.github/workflows/publish.yml

@@ -13,8 +13,7 @@ jobs:
         os:
           [
             { name: "linux-x86_64", runner: "ubuntu-latest" },
-            { name: "macos-x86_64", runner: "macos-latest" },
-            { name: "macos-aarch64", runner: "macos-latest-xlarge" },
+            { name: "macos-aarch64", runner: "macos-latest" },
           ]
     runs-on: ${{ matrix.os.runner }}
     permissions: