[perlgeek]

> What is this rule to only build once?

If your cycle is "build, test, build, deploy", then you are not deploying those artifacts that you tested.

Any number of factors (different dependency version, toolchain difference, environment differences, non-reproducible builds) could lead to the second build being different from the first one, and then you deploy an untested artifact.

Not to mention that rebuild can be resource intenstive.