update to latest macos release (GH actions are removing 10.14).
Explicitly update the toolchain; some instances are still on 1.38
but the current version is 1.39.
Forgetting to update the submodules is a commonly reported build
issue with an obscure error message:
```
CMake Error: The source directory "/home/USER/wezterm/target/debug/build/freetype-430fe24cb64c561c/out/zlib-src/zlib" does not appear to contain CMakeLists.txt.
```
this teaches the build.rs machinery to run the submodule update
if it looks like it is needed.