find_package(OpenColorIO) broken under cmake 3.29.1

With the recent update of cmake to v3.29.1 (v3.29 was okay before) of GitHub Actions, OpenColorIO started failing not finding its dependencies:
```
CMake Warning (dev) at /opt/homebrew/Cellar/cmake/3.29.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:438 (message):
The package name passed to `find_package_handle_standard_args` (EXPAT) does
not match the name of the calling package (expat). This can lead to
problems in calling code that expects `find_package` result variables
(e.g., `_FOUND`) to follow a certain pattern.
Call Stack (most recent call first):
/opt/homebrew/Cellar/cmake/3.29.1/share/cmake/Modules/Findexpat.cmake:114 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
/opt/homebrew/Cellar/cmake/3.29.1/share/cmake/Modules/CMakeFindDependencyMacro.cmake:76 (find_package)
build/install/lib/cmake/OpenColorIO/OpenColorIOConfig.cmake:33 (find_dependency)
CMakeLists.txt:163 (find_package)
This warning is for project developers. Use -Wno-dev to suppress it.

-- Found EXPAT: /Users/runner/work/tlRender/tlRender/build/install/lib/libexpat.a (found version "2.5.0")
CMake Error at CMakeLists.txt:163 (find_package):
Found package configuration file:

/Users/runner/work/tlRender/tlRender/build/install/lib/cmake/OpenColorIO/OpenColorIOConfig.cmake

but it set OpenColorIO_FOUND to FALSE so package "OpenColorIO" is
considered to be NOT FOUND. Reason given by package:

OpenColorIO could not be found because dependency expat could not be found.

-- Configuring incomplete, errors occurred!
```

On macOS it fails on expat. On Linux it fails with pystring.

Inizia riproducendo il fallimento di configurazione di GitHub Actions con CMake 3.29.1 in CMakeLists.txt:163. Ispeziona build/install/lib/cmake/OpenColorIO/OpenColorIOConfig.cmake e confronta il fallimento di expat su macOS con il fallimento di pystring su Linux. Il lavoro è completato quando find_package(OpenColorIO) viene configurato correttamente su entrambe le piattaforme.

Scritto dal modello di indicizzazione a partire dal testo della issue.