Fix building C# projects in Godot (#3778)

Since version 3.0 Godot is supporting C# as a language for writing
scripts. The C# solution can be built directly in Godot editor using
MSBuild, which requires access to directory /etc/mono. This directory
contains configuration of Mono enviroment. If MSBuild don't have
access to this directory, it's not able to determine location of
DLL files and it's throwing System.DllNotFoundException at beginning
of the build process.

etc/profile-a-l/godot.profile

@@ -38,7 +38,7 @@ tracelog
 # private-bin godot
 private-cache
 private-dev
-private-etc alsa,alternatives,asound.conf,ca-certificates,crypto-policies,drirc,fonts,machine-id,nsswitch.conf,openal,pki,pulse,resolv.conf,ssl
+private-etc alsa,alternatives,asound.conf,ca-certificates,crypto-policies,drirc,fonts,machine-id,mono,nsswitch.conf,openal,pki,pulse,resolv.conf,ssl
 private-tmp
 
 dbus-user none