fix: decompress packages.gz for completion cache

[pfeifferj]

AUR changed how packages.gz is served to no longer include the Content-Encoding: gzip header, so HTTP clients no longer auto-decompress the response. This caused raw gzip binary data (including null bytes) to be written to the completion cache, resulting in bash warnings: "command substitution: ignored null byte in input"

Fixes: #1472, #1450

[pfeifferj]

related discussion: https://gitlab.archlinux.org/archlinux/aurweb/-/issues/546#note_364572

[4yx]

Is this any different from #1449?

[pfeifferj]

oh I guess #1447 is a third duplicate issue for this that I missed