security/easy-rsa: fix cert issuance with BSD grep

easyrsa running on systems with bsdgrep for grep
fails issuing certs because it attempts \d as shorthand for
:digit: or [0-9] and triggers a grep failure with diagnostic

grep: trailing backslash (\)

Filed upstream: https://github.com/OpenVPN/easy-rsa/issues/556

PR: 263812
Submitted by: grembo@