This lets us benefit from the recent OpenSSL security-related
update [1]. Since nixos-unstable is still stuck, we temporarily
use nixos-unstable-small as our unstable channel.
Fixes necessary:
* //users/sterni/nix/char:
Someone has decided to drop writers.writeC upstream [2],
so we reimplement it ad-hoc using runCommandCC
[1]: https://www.openssl.org/news/secadv/20210824.txt
[2]: 982f46985e
Change-Id: Id84756e2e370296b7a27e1a3f1744f58f8fe3c47
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3463
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI