Fix cleanup of nightly built ISO images

http://nightly.tails.boum.org/build_Tails_ISO_experimental/ and http://nightly.tails.boum.org/build_Tails_ISO_devel/ only have artifacts for the last few days. In case it matters, these branches didn’t build for a few days, so perhaps the script expects more files and was a little lost?

Feature Branch: puppet-tails:bugfix/6439-fix-cleanup-of-nightly-built-ISO-images

Original created by @intrigeri on 6439 (Redmine)

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