Ensure umask is consistent with the comparison blobs

Debian's default umask is 0022 whilst Ubuntu's is 0002. This difference results in different file modes appearing in the debs generated during the unshare test under Ubuntu; adjust the umask prior to creating the test source to ensure things pass.

This change isn't strictly necessary for Debian, but would eliminate the last bit of Ubuntu's delta with Debian's sbuild.

On the question of why Debian and Ubuntu have different umasks: despite both setting USERGROUPS_ENAB to "yes" in /etc/login.defs, which implies that the group mode of the umask will be copied from the user mode, it appears only Ubuntu honours this setting because Debian doesn't include pam_umask.so in its PAM common-session configuration. I'm not sure if this is intentional or an issue in and of itself (I haven't tracked down the ultimate source of those files) but it would appear this is the root of the difference between the distributions.