Change of default value for compress_tar makes test.pl fail

After changing default value for compress_tar in a91d4653, test.pl fails with:

ok 38 - mercurial repo
Created /tmp/rbm-C8aGQO/rbm-ZxVr4/mozmill-automation-bbad7215c713.tar.gz
sha256sum: mozmill-automation-bbad7215c713.tar: No such file or
directory
Error: Error running build
# Looks like your test exited with 1 just after 38.

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