Use a fixed macos version (#2498)

.github/workflows/build.yml

@@ -60,9 +60,13 @@ jobs:
             ocamlparam: '_,Oclassic=1'
             disable_testcases: 'ocaml/testsuite/tests/typing-local/regression_cmm_unboxing.ml ocaml/testsuite/tests/int64-unboxing/test.ml'
 
+          # CR ccasinghino: We encountered build errors (missing autoconf, and
+          # then other errors) when `macos-latest` started referring to version
+          # 14.  So we've put a fixed version below for now, but we should
+          # eventually fix this.
           - name: flambda2_macos
             config: --enable-middle-end=flambda2 --disable-warn-error
-            os: macos-latest
+            os: macos-12
 
           - name: irc
             config: --enable-middle-end=flambda2