Adds an extra check to determine if the placeholder ocurrs in template
content before actually reading the corresponding secret file.
In terms of performance, this adds an extra string search, but removes
possibly unneceassary file reading if the secret is not used in the
template, though both of them should be negligible in most cases.
FixesMic92/sops-nix#496.
When using `documentation.nixos.includeAllModules = true;` this fails
for me with
error: attribute 'users' missing
at /nix/store/30jax2y4q3d1xyf8ha3cwilw3kdz4pn8-source/modules/sops/templates/default.nix:8:11:
7| secretsForUsers = lib.filterAttrs (_: v: v.neededForUsers) cfg.secrets;
8| users = config.users.users;
| ^
9| in {
(use '--show-trace' to show detailed location information)
Not depending on `config` when building the manual fixes the issue.
