CI: attempt to upgrade Mac OS X deployment target to 10.12. (#11787)

Reading through the perplexing release build failure for `macos-x86_64`
in [1], I noticed a warning stating the following:

```
warning: deployment target in MACOSX_DEPLOYMENT_TARGET was set to 10.9, but the minimum supported by `rustc` is 10.12
```

I'm not sure if this has anything to do with the new build failure, but
it seems worth addressing in any case. This PR updates the minimum Mac
OS X version targeted by the build to 10.12.

[1]: https://github.com/bytecodealliance/wasmtime/actions/runs/18228566641/job/51906071185#step:8:641

Author: cfallin

ci/build-build-matrix.js

@@ -42,14 +42,14 @@ const array = [
     "os": macos,
     "target": "x86_64-apple-darwin",
     // On OSX all we need to do is configure our deployment target as old as
-    // possible. For now 10.9 is the limit.
-    "env": { "MACOSX_DEPLOYMENT_TARGET": "10.9" },
+    // possible. For now 10.12 is the limit.
+    "env": { "MACOSX_DEPLOYMENT_TARGET": "10.12" },
   },
   {
     "build": "aarch64-macos",
     "os": macos,
     "target": "aarch64-apple-darwin",
-    "env": { "MACOSX_DEPLOYMENT_TARGET": "10.9" },
+    "env": { "MACOSX_DEPLOYMENT_TARGET": "10.12" },
   },
   {
     "build": "x86_64-windows",