nixos/murmur: Harden /proc visibility by setting ProcSubset to pid

https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#ProcSubset=

Tested that the Mumble server still works and clients can connect to it.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

nixos/modules/services/networking/murmur.nix

@@ -386,6 +386,7 @@ in
         PrivateMounts = true;
         PrivateTmp = true;
         PrivateUsers = true;
+        ProcSubset = "pid";
         ProtectClock = true;
         ProtectControlGroups = "strict";
         ProtectHome = true;