Draft: abuild: die on failing to cd to builddir

failing to cd into the builddir is usually almost always an error that is just harder to debug than it needs to be.

for example, upon failing to cd before build(), the directory will be in srcdir, and then the build() will give confusing results, until you realise cwd is just not where you expect it.

things that rely on an implicit cwd/srcdir should explicitly set it, with builddir="$srcdir" or similar. then it's obvious what they expect

there aren't many cases of this in aports, and they would be easy to fix (with clear errors) where they were relied on