olive-editor: fix build of pinned openimageio

- add `override` to pinned `openimageio` with `fmt_10`, it does not
build with new default `fmt_11`

Fixes build failure of pinned `openimageio`:
```
CMake Error at src/cmake/modules/Findfmt.cmake:19 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  src/cmake/externalpackages.cmake:325 (find_package)
  src/cmake/externalpackages.cmake:354 (find_or_download_fmt)
  CMakeLists.txt:166 (include)
```