Drop x86_64-macos from releases (GitHub retired the Intel runner)

The macos-13 Intel runner no longer gets allocated, so the x86_64-macos release
job queues forever. Ship prebuilt binaries for x86_64-linux and aarch64-macos;
Intel Macs build from source. The install script now says so instead of 404ing
on a missing asset.

.github/workflows/release.yml

@@ -26,8 +26,8 @@ jobs:
         include:
           - os: ubuntu-latest
             target: x86_64-linux
-          - os: macos-13
-            target: x86_64-macos
+          # No x86_64-macos: GitHub is retiring the macos-13 Intel runner (jobs
+          # queue forever). Intel Macs build from source. macos-14 is arm64.
           - os: macos-14
             target: aarch64-macos
     steps: