bug#35012: Can't install icecat

[Danny Milosavljevic]

Just adding a few suggestions for Guix to do:

> Especially for big packages like icecat there can always be a gap
> between the time when the package definition is updated and when a
> substitute is built and ready to be downloaded.

Especially since icecat depends on Rust now.  Rust has a very long
bootstrap chain and if we change something in the base, the chain
has to be rebuilt.  That takes days on current hardware.

Also, currently, we disable tests parallelization in Rust 1.20 because
they have an error there.  But it would be possible and desireable to
enable them for newer versions again and see whether they work.  That
would entail replacing the "check" phase by one which invokes ./x.py
with

  "-j" (number->string (parallel-job-count))

(instead of the current

  "-j1"
)

And then build like 10 rounds.

If it works, the build of each of the corresponding Rust derivations
would be faster.

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