blessed-builds: lintian check always fails

the lintian check checks the binary package but that will always fail since the .dsc got mangled by our mergechanges call:

E: mesa changes: bad-distribution-in-changes-file amber-phone
E: mesa changes: file-size-mismatch-in-changes-file mesa_19.2.4-1pureos+librem5.2.dsc 4524 != 5233
E: mesa changes: checksum-mismatch-in-changes-file md5 mesa_19.2.4-1pureos+librem5.2.dsc
E: mesa changes: checksum-mismatch-in-changes-file sha1 mesa_19.2.4-1pureos+librem5.2.dsc
E: mesa changes: checksum-mismatch-in-changes-file sha256 mesa_19.2.4-1pureos+librem5.2.dsc

@arno.bauernoppel to avoid that you can either change it to check the source changes file or (better) add --source-only-changes to pbuilder-options and drop the mergechangelog call completely.