It is a common mistake to modify an <origin> record that computes to a
fixed-output derivation, forgetting to update its hash field, only to find out
that a previously built source gets used instead. While there doesn't seem to
be an obvious solution to this problem, we can at least document the fact that
sources can be checked the same as other builds.
* doc/guix.texi (Additional Build Options): Document that source derivation
builds can be verified using --check.
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