drop support for environment variables

The configuration file is now the canonical way to, well, configure.
This change is desirable because it gives us much more flexibility with
how configuration is structured. Environment variables are insufficient
because, for example, they're a flat namespace and have no built-in way
to represent lists.

nix/modules/default/default.nix

@@ -25,15 +25,6 @@ in
         pkgsText = "inputs.grapevine.packages.\${pkgs.system}";
       };
 
-    extraEnvironment = lib.mkOption {
-      type = types.attrsOf types.str;
-      description = ''
-        Extra environment variables to set for the process.
-      '';
-      default = {};
-      example = { RUST_BACKTRACE="yes"; };
-    };
-
     settings = lib.mkOption {
       type = types.submodule {
         freeformType = format.type;
@@ -85,7 +76,6 @@ in
     systemd.services.grapevine = {
       description = "Grapevine (Matrix homeserver)";
       wantedBy = [ "multi-user.target" ];
-      environment = cfg.extraEnvironment;
 
       # Keep sorted
       serviceConfig = {

src/main.rs

@@ -17,7 +17,7 @@ use axum_server::{
     bind, bind_rustls, tls_rustls::RustlsConfig, Handle as ServerHandle,
 };
 use figment::{
-    providers::{Env, Format, Toml},
+    providers::{Format, Toml},
     Figment,
 };
 use http::{
@@ -111,9 +111,7 @@ async fn try_main() -> Result<(), error::Main> {
     let args = args::parse();
 
     // Initialize config
-    let raw_config = Figment::new()
+    let raw_config = Figment::new().merge(Toml::file(&args.config).nested());
-        .merge(Toml::file(&args.config).nested())
-        .merge(Env::prefixed("GRAPEVINE_").global());
 
     let config = raw_config.extract::<Config>()?;