systemd: Properly escape environment options.

Using toJSON on a string value works because the allowed JSON escape
sequences is almost a subset of the systemd allowed escape sequences.
The only exception is `\/` which JSON allows but systemd doesn't.
Luckily this sequence isn't required and toJSON don't produce it making
the result valid for systemd consumption.

Examples of things that this fixes are environment variables with double
quotes or newlines.

nixos/modules/system/boot/systemd.nix

@@ -329,7 +329,7 @@ let
           ${let env = cfg.globalEnvironment // def.environment;
             in concatMapStrings (n:
               let s = optionalString (env."${n}" != null)
-                "Environment=\"${n}=${env.${n}}\"\n";
+                "Environment=${builtins.toJSON "${n}=${env.${n}}"}\n";
               in if stringLength s >= 2048 then throw "The value of the environment variable ‘${n}’ in systemd service ‘${name}.service’ is too long." else s) (attrNames env)}
           ${if def.reloadIfChanged then ''
             X-ReloadIfChanged=true