Fix re-downloading tarball when destination file already exists

Previously, curl and wget were instructed to try to resume the download
if the destination file already exists. This is supposed to be done via
the "Range" HTTP header, but doesn't work well with CloudFront:

    HTTP server doesn't seem to support byte ranges. Cannot resume.

CloudFront is supposed to support ranges, so I don't know what's going
on here. It might be failing only in case the existing file is a fully
downloaded tarball?

In any case, this disables resuming downloads and resorts to simply
re-downloading the tarball always, overwriting the existing file.

Fixes #487
Avoid re-downloading if there's a valid tarball in build location

If there already exists a valid tarball in the build location, e.g. as
artefact of a previous install using `--keep`, don't re-download the
file, as there is no need.

References #487
Merge branch 'no-resume-download'