Enforce correct attributes for acme.sh private keys

Summary:
Apply logic from 421712d5da56 to private key files too, so several applications
can access the certificate private key they need.

Ref T2132

Test Plan: When run on Hervil, confirmed no-op as keys are already 640/600.

Reviewers: DorianWinty

Maniphest Tasks: T2132

Differential Revision: https://devcentral.nasqueron.org/D3732