fim: Fix the build by switching back to GCC 9
fim: Fix the build by switching back to GCC 9

GCC 10 breaks the build: https://hydra.nixos.org/build/134272249
There are workarounds to fix the build with GCC 10 [0] but since
upstream is inactive it's probably best to deprecate fim soon.

[0]: https://aur.archlinux.org/cgit/aur.git/commit/?h=fim&id=66258387a099da72f7758495fb0e3c93e16068e7

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