packer/README: Update required install step
Otherwise, packer from debian 1.3.4 is too old to work
Test Plan
- packer validate debian_buster.json: fails with packer 1.3.4
- remove packer from debian
- install new repository as described in the diff (packer ~ 1.6)
-
packer validate debian_buster.json
ok
Migrated from D4093 (view on Phabricator)