keg: fix normalize_pod2man_outputs! for compressed manpages

[alebcay]

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Attempting to read gzipped manpages as text results in a cryptic error ArgumentError: invalid byte sequence in UTF-8. A future enhancement would be to decompress compressed manpages to perform the same operation and then re-compress them (or perhaps just reproducibly compress all manpages).

Fixes #19400

[MikeMcQuaid]

Thanks!

[caarlos0]

I think this might be causing #19400