bug#29655: Elixir build has an undeterministic build error

[nee]

Hello, this has already mentioned in another bug report
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28034#20

The built of Elixir sometimes fails while compiling
'lib/elixir/lib/system.ex' with the error message:

> ** (exit) :epipe

See appended build log. I found the only other mention of the error on
the guix mailing list when elixir was first added:

https://lists.gnu.org/archive/html/guix-devel/2016-07/msg01436.html
https://lists.gnu.org/archive/html/guix-devel/2016-08/msg00114.html
https://lists.gnu.org/archive/html/guix-devel/2016-07/msg01540.html

It was suspected to be related to high memory usage / getting into swap.
I ran the build on two computers with 6GB and 16GB ram and still got it
on both, so I don't think it's ram related.

* What it might be:
One single time I got a different error message for the same file which
mentioned git (see detailed error attachment).
The file lib/elixir/lib/system.ex has a macro that calls git. Also this
bug seems to appear nowhere outside of guix, so it might be related.

error-elixir-generic-epipe.log
Description: Text Data

error-elixir-detailed-git.log
Description: Text Data