Murmur can't read generated acme keys in 20.09

[pstn]

Describe the bug

• The acme module lost the abitlity to set a generated certs/keys uid, only sets gid since Restructure acme module #91121 (review)
• The murmur module only runs with nogroup

Expected behavior
The modules should fit together, preferably by the murmur module setting a proper gid.

Notify maintainers

@petabyteboy
@infinisil

@NixOS/acme

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.58, NixOS, 20.03.3113.d74573d8ae6 (Markhor)`
 - multi-user?: `yes`
^[[B^[[B^[[B - sandbox: `yes`
 - version: `nix-env (Nix) 3.0pre20200829_f156513`
 - channels(root): `"nixos-20.09.1469.13d0c311e3a"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: [ services.murmur, security.acme ]

[lheckemann]

+1 for giving murmur a proper gid, stuff definitely shouldn't be running as nogroup anyway.