Vultr DNS: fix "grep: repetition-operator operand invalid" on FreeBSD

[skyksandr]

@mjkrg reports here https://github.com/acmesh-official/acme.sh/issues/2374 integration is no longer working on FreeBSD. This PR aims to fix it.

On line 119, containing the command grep '^\{.*\}', on FreeBSD it needs an -E option to be able to parse the extended regular expression otherwise it chokes with the error message: "grep: repetition-operator operand invalid".

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[Neilpang]

https://github.com/skyksandr/acme.sh/actions/runs/2916012688

[skyksandr]

@Neilpang Here is CI run after rebasing on current master: https://github.com/skyksandr/acme.sh/actions/runs/4081483447