firmware: save the compressed contents file to disk instead of decompressed

the existing logic was to decompress the contents file from the downloaded
archive to disk, then process it to obtain a package list. the largest
one by far is for 'main'; 'non-free' and 'contrib' are tiny in comparison.
for sid-amd64 currently, the archive file is 37 MB, while the decompressed
file it contains is 592.3 MB.

we always delete the files and download afresh (currently), and a previous
commit optimised by deleting the files once we're done with them to avoid
wasting disk space leaving them behind.

here we switch to storing the downloaded compressed file to disk instead,
reducing disk space usage (and IO) by hundreds of megabytes; piping the
decompression directly into awk instead of having awk read from the stored
file.

this moves the appending of new items into the list back within the archive
area loop, which is fine since we're replacing the file for each loop now
so the previous issue relating to appending is of no concern.

Gbp-Dch: Short
Closes: #952910

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