[*] postinstall: Don't try to link cloud specs if pkgs don't exist
[*] postinstall: Don't try to link cloud specs if pkgs don't exist

Summary: see title

Test Plan: manual

Reviewers: evan, spang, mark, halla

Reviewed By: halla

Differential Revision: https://phab.nylas.com/D4446

simonft/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas-mail-lives/nylas-mail