We need to build in release mode so targets are cacheable.
GH has a limit of 400MB per cache blob and we're at 750MB with
the debug build. Slim it down a bit.
The cache wants to hash the Cargo.lock file so I've removed that
from the ignore file and added it to the repo. This might cause
an error for users checking out the repo after this change is
pushed; removing the local Cargo.lock should resolve that.
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.
