nixos/prometheus/alertmanager: use ExecStart instead of script

This results in a simpler service unit which doesn't first have to
start a shell:

  > cat /nix/store/s95nsr8zbkblklanqpkiap49mkwbaq45-unit-alertmanager.service/alertmanager.service
  ...
  ExecStart=/nix/store/4g784lwcy7kp69hg0z2hfwkhjp2914lr-alertmanager-0.16.2-bin/bin/alertmanager \
    --config.file /nix/store/p2c7fyi2jkkwq04z2flk84q4wyj2ggry-checked-config \
    --web.listen-address [::1]:9093 \
    --log.level warn
  ...