net-mgmt/alertmanager: Fix logging

Redirect stdout and stderr of the Alertmanager to its log file with
the daemon(8) -o flag instead of shell redirections. The reason for
this change is that shell redirections simply do not work in this case.

While here, remove an unnecessary env(1) invocation.

Approved by: portmgr blanket (runtime fix)
Fixes: df292e75acbe
MFH: 2022Q1
Sponsored by: Ekco
Sponsored by: Klara Inc