meson: Add some missing env settings for tests of pg_dump and pg_verifybackup

meson: Add some missing env settings for tests of pg_dump and pg_verifybackup

The commands used for the compression tests were missing in a few
places, causing the tests related to these to never run.

Georgios has spotted GZIP_PROGRAM missing in pg_dump, while I have
noticed the ones missing in pg_verifybackup while looking at the rest of
the tree.

Author: Georgios Kokolatos, Michael Paquier
Discussion: https://postgr.es/m/O4mutIrCES8ZhlXJiMvzsivT7ztAMja2lkdL1LJx6O5f22I2W8PBIeLKz7mDLwxHoibcnRAYJXm1pH4tyUNC4a8eDzLn22a6Pb1S74Niexg=@pm.me