Change of default value for compress_tar makes test.pl fail
After changing default value for `compress_tar` in
a91d4653cb5f66d86fd4d306c564161acfd6fa79, `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.
```
issue