scripts: fix finishing the toolchain when download/extract-only is set
In case we only download or extract the sources, do not fail while
finishing the toolchain: the test-suite directory may not exist, so
we can't chmod it.
Also, use safer constructs that won't trigger the 'set -e' in case of
failure (eg.: "[ ... ] && ..." is not safe in case the test fails).
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>