Enable verification of langpacks checksums
When switching to esr60 we temporarily disabled the checking of langpacks checksums in
projects/firefox-langpacks/build because they were missing in the SHA512SUMS files.
It looks like the bug causing this issue will be fixed soon: https://bugzilla.mozilla.org/show_bug.cgi?id=1463749
We should enable this check again in the next release where this bug has been fixed.