nixos/peertube: startup script: exec into nodejs

1. Removed the #!/bin/sh shebang at the beginning, because
systemd.services.<name>.script already adds a #!/nix/store/.../bin/bash
shebang.

Previously:
	#!/nix/store/516kai7nl5dxr792c0nzq0jp8m4zvxpi-bash-5.2p32/bin/bash
	set -e
	#!/bin/sh
	umask 077
	...

2. Exec into nodejs, so that the startup script is no longer running but
replaces itself by nodejs.
This way, only one processus is running inside peertube.service.

nixos/modules/services/web-apps/peertube.nix

@@ -432,7 +432,6 @@ in {
       path = with pkgs; [ nodejs_18 yarn ffmpeg-headless openssl ];
 
       script = ''
-        #!/bin/sh
         umask 077
         cat > /var/lib/peertube/config/local.yaml <<EOF
         ${lib.optionalString (cfg.secrets.secretsFile != null) ''
@@ -457,7 +456,7 @@ in {
         ln -sf ${cfg.package}/config/default.yaml /var/lib/peertube/config/default.yaml
         ln -sf ${cfg.package}/client/dist -T /var/lib/peertube/www/client
         ln -sf ${cfg.settings.storage.client_overrides} -T /var/lib/peertube/www/client-overrides
-        node dist/server
+        exec node dist/server
       '';
       serviceConfig = {
         Type = "simple";