Put back npm --save in installDeps.sh

[muxator]

We may need to activate again npm --save in installDeps.sh. Contextually, we may want to start tracking package-lock.json.

This would revert c4918ef, and partially negate what was done for #3396, but aligns better with current practices in the nodejs ecosystem.

Pragmatically speaking, this will allow users - if they want - to use npm-force-resolutions to manually fix security vulnerabilities.

We had a problem for that (see #3598), and given the fragmented nature of the
nodejs ecosystem it is reasonable to expect more issues like that one, so I think it's better to be prepared.