github: try building on macos-13 (x86)

We all noticed that x86 macOS binaries are no longer being provided on release,
due to `macos-11` runners going the way of the Dodo a while back. Nobody
alterted us to this, funny enough.

After some quick discussion, we concluded some things:

- x86 macOS runners are likely oversubscribed, and hurt CI latency badly
- `macos-12` is also deprecated; `macos-13` is the best x86 runner available
- GitHub probably isn't going to expand macOS runner capacity; `macos-13` will
one day go away
- Some people are still using `jj` on Intel Macs. We didn't get alerted because
they do their own builds for now, but may not always do that.
- We can just try to build on `macos-13` and make it optional for merges.

So that's what this does. It might be mergeable outright, but we can also use it
to measure build latency impacts.

Signed-off-by: Austin Seipp <[email protected]>

.github/workflows/build.yml

@@ -15,7 +15,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-14, windows-latest]
+        # macos-13 is x86; macos-14 is ARM
+        os: [ubuntu-latest, macos-13, macos-14, windows-latest]
         cargo_flags: [""]
         include:
         - os: ubuntu-latest