build(ci): ensure correct headers are used on macOS

Currently, the release build picks up headers in
`/Library/Frameworks/Mono.framework/Headers`. You can verify this by
downloading the latest nightly build and checking the output of `nvim
--version`.

These headers are likely to be from a different version of `libintl` than the
one we link to. Let's avoid usage of them by setting `CMAKE_FIND_FRAMEWORK` to
`NEVER`.

.github/workflows/test.yml

@@ -134,6 +134,8 @@ jobs:
             flags: -D UNSIGNED_CHAR=ON
           - cc: clang
             runner: macos-12
+            flags: -D CMAKE_FIND_FRAMEWORK=NEVER
+            deps_flags: -D CMAKE_FIND_FRAMEWORK=NEVER
 
             # functionaltest-lua is our dumping ground for non-mainline configurations.
             # 1. Check that the tests pass with PUC Lua instead of LuaJIT.