Fix rustfmt checking files twice

[MikkelPaulson]

Apparently rustfmt tests the entire code tree when pointed at lib.rs, so all files were being checked twice (except lib itself, I guess). The git ls-files song and dance was unnecessary, just point it at {lib,main}.rs and let 'er rip.

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[MikkelPaulson]

$ git ls-files '*.rs' | wc -l
139
$ rustfmt --check --edition 2021 --verbose */src/{lib,main}.rs */tests/*.rs | grep 'rs$' | wc -l
139