Add POTFILES.in updater check to linter CI job.

while processing mana/plus!132 I spotted what looked like an ommision of several
src/progs/{manaplus,manaverse} files from POTFILES.in, but as it turned
out, they have no strings marked for translation (start with "_(").

Squashed with:
* Just force LC_COLLATE=C and commit POTFILES.in changes required to make
this happen. Don't know how to repro CI's sorting locally and it's
pointless as my local sort is clearly wrong. See traderecv.cpp's position
for why.

* The correct locale variable to set should be LC_COLLATE, which is
probably set by LANG on all platforms and so this will fail, again.

* sort is still not preserving the order of files :/

* Ensure the order of files remains the same regardless of platform.

* POTFILES update to pass CI :)

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mana/plus!133

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