build: say so when apt-get update fails instead of passing over it

34cb6e6 added `apt-get update;` before the install. The semicolon means
a failed update does not stop the install, which is the behaviour we
want -- one unreachable repository should not block an install the
lists already on disk can satisfy -- but it was punctuation rather than
a decision, and it was silent. The install then failed several seconds
later with "unable to locate package" and nothing tied the two
together.

Kept it non-fatal, made it explicit with `|| echo`, and wrote the
reasoning down next to it. The install itself is still the last command
in the branch, so a failed install is still the target's status.

Measured all four combinations with apt-get shimmed: a failed update
now warns and carries on, a failed install still aborts.

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