build.sh: keep a copy of the previous build.log for reference
build.sh: keep a copy of the previous build.log for reference

This should be quite useful specially when looping with run-nightly.sh
since otherwise the old log file is immediately lost.

We might consider later if we want to keep more logs around,
though with nightly they would build up quite fast

[skip ci]

jonschoning/stackage,mrkkrp/stackage,simonmichael/stackage