Turns out the build time only seems to capture the last time
you managed to force a full rebuild (???). Even though cargo
indicates that a rebuild had happened of the main.rs, the environment
variable containing the time is not being updated, because the
build.rs file isn't being run. But! if you force a trigger of
that file, then cargo decides to rebuild *everything* which greatly
increases the build time, which is a drag for development.
So, just remove the build time notice. If it's not going to be
correct, better to not emit it than to be outright misleading...
