add XBPS_SYSLOG environment variable to overwrite configuration

bin/xbps-install/xbps-install.1

@@ -245,6 +245,11 @@ This variable differs from
 in that it allows you to install packages partially, because
 configuration phase is skipped (the target binaries might not be compatible with
 the native architecture).
+.It Sy XBPS_SYSLOG
+Overrides the
+.Xr xbps.d 5
+.Sy syslog=true|false
+configuration option.
 .El
 .Sh FILES
 .Bl -tag -width /var/db/xbps/.<pkgname>-files.plist