bug(linux): Keyman keyboards don't replace characters when used with fcitx5

When using fcitx5 instead of ibus (and selecting fcitx5 as the "Keyboard input method system" in Settings/Region & Language/Manage Installed Languages), it seems that keyboards that replace characters don't work properly.

For example "IPA (SIL)": typing `n>` should result in `ŋ`, but actual result is `n>`.
"Khmer Angkor":
- typing `xjmEr` gives correct result `ខ្មែរ`
- typing `xEjmr` gives wrong result `ខែ្មែ្រ`

This happens with both X11 and Wayland mode.