Cmake error when building openColorIio due to missing Imath.pc

I'm getting an error when Cmake tried to build openColorIo as it can't find Imath.pc.

C:/Development/XStudio/xstudio/build/vcpkg/downloads/tools/msys2/9272adbcaf19caef/mingw64/bin/pkg-config.exe
17:25:12:654 1> [CMake] --exists OpenColorIO failed with error code: 1
17:25:12:654 1> [CMake]
17:25:12:654 1> [CMake] ENV{PKG_CONFIG_PATH}: "C:/Development/XStudio/xstudio/build/vcpkg/packages/opencolorio_x64-windows/lib/pkgconfig;C:/Development/XStudio/xstudio/build/vcpkg/packages/opencolorio_x64-windows/share/pkgconfig;C:/Development/XStudio/xstudio/build/vcpkg_installed/x64-windows/lib/pkgconfig;C:/Development/XStudio/xstudio/build/vcpkg_installed/x64-windows/share/pkgconfig"
17:25:12:654 1> [CMake] output: Package Imath was not found in the pkg-config search path.
17:25:12:654 1> [CMake]
17:25:12:654 1> [CMake] Perhaps you should add the directory containing `Imath.pc'
17:25:12:654 1> [CMake]
17:25:12:654 1> [CMake] to the PKG_CONFIG_PATH environment variable
17:25:12:654 1> [CMake]
17:25:12:654 1> [CMake] Package 'Imath', required by 'OpenColorIO', not found
17:25:12:654 1> [CMake] Call Stack (most recent call first):
17:25:12:654 1> [CMake] scripts/cmake/vcpkg_fixup_pkgconfig.cmake:196 (z_vcpkg_fixup_pkgconfig_check_files)
17:25:12:654 1> [CMake] buildtrees/versioning_/versions/opencolorio/661b43b1d2c452e40476bbb64bb6e68dfc437c7b/portfile.cmake:68 (vcpkg_fixup_pkgconfig)
17:25:12:654 1> [CMake] scripts/ports.cmake:206 (include)

Tried adding imath as a dependency in vcpkg.json, but no dice.

Start with vcpkg.json and the OpenColorIO portfile at buildtrees/versioning_/versions/opencolorio/661b43b1d2c452e40476bbb64bb6e68dfc437c7b/portfile.cmake, then inspect scripts/cmake/vcpkg_fixup_pkgconfig.cmake around the reported call. Reproduce the Windows build and trace why Imath.pc is absent from PKG_CONFIG_PATH; done means the OpenColorIO port configures without the missing-package error.