make fcitx5-diagnose.sh able to handle the situation when env is set to fcitx5

[karuboniru]

Don't raise confusing error message like

    **Environment variable GTK_IM_MODULE is "fcitx5" instead of "fcitx". Please check if you have exported it incorrectly in any of your init files.**
    **You may have trouble using fcitx in gtk programs.**

    **Please set environment variable GTK_IM_MODULE to "fcitx" using the tool your distribution provides or add `export GTK_IM_MODULE=fcitx` to your `~/.xprofile`. See [Input Method Related Environment Variables: GTK_IM_MODULE](http://fcitx-im.org/wiki/Input_method_related_environment_variables#GTK_IM_MODULE).**

When those environment variable is set to fcitx5

[wengxt]

No. we want people to keep using fcitx.

[WhiredPlanck]

So fast ......