Previously, 'guix substitute' would fail abruptly with something like:
guix substitute: warning: while fetching https://ci.guix.gnu.org/nar/lzip/…-example: server is somewhat slow
guix substitute: warning: try `--no-substitutes' if the problem persists
guix substitute: error: connect*: Connection timed out
substitution of /gnu/store/…-example failed
* guix/scripts/substitute.scm (network-error?): Add ETIMEDOUT.
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