upload-update-site.sh: detect executable to use

The Bio-Formats update site Jenkins jobs failed today because
ImageJ-linux64 was missing, even though fiji-linux64 was still present.
Probably there was a change in the Fiji update site configuration. But
regardless, let's be more thorough about detecting which executable to
use for uploading the update site, rather than hardcoding 64-bit Linux.
upload-update-site.sh: add check for launcher

If the launcher is not present, fail fast.
upload-update-site.sh: remove bashism

It seems that double equals is bash-specific, whereas single equals is
OK. I probably knew this once or twice, but forgot... again.
upload-update-site.sh: do not use "fiji" launchers

The reason is that the fiji launchers just hand off to the ImageJ
launcher anyway. So if the ImageJ launcher is not present, they will
fail regardless.
upload-update-site.sh: download Fiji earlier

If Fiji has not been downloaded yet, we should download it, *then* check
for the ImageJ launcher. Otherwise it will always be missing, of course.
upload-update-site.sh: fix test for Fiji dir

The test was inverted.