[Backport MB-48190 to 6.6.4] Set environment variable GODEBUG=madvdontneed=1 for indexer

Due to the issues seen in MB-47600 and MB-44731, the indexer's golang version upgrade(go1.13.7->go1.16.5) for 7.0.2 has been reverted.

The golang upgrade was also required to address the high memory usage issue seen in MB-46323.

As the upgrade is not possible now, the high memory usage issue needs to be addressed via setting the following environment variable when indexer is started on command line.

GODEBUG=madvdontneed=1

As ns-server spawns the indexer process, this change needs to be done in ns-server code.

For reference, ns-server already has the code to set the GOTRACEBACK env variable for indexer:
https://github.com/couchbase/ns_server/blob/master/src/ns_ports_setup.erl