Allow to read env vars from files inside the "env.d" directory

This makes it easier to set environment variables on some operating systems.
Setting configuration options from environment variables is recommended if
you want to avoid the time-consuming task of merging your changes with the
default configuration file after upgrading SFTPGo

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

pkgs/build.sh

@@ -93,6 +93,9 @@ contents:
   - dst: "/var/lib/sftpgo"
     type: dir
 
+  - dst: "/etc/sftpgo/env.d"
+    type: dir
+
 overrides:
   deb:
     recommends: