fix: allow overriding npm_ env vars via env

[legobeat]

Allow user to override the following env vars via env params:

• npm_package_json
• npm_lifecycle_event
• npm_lifecycle_script

Package defaults still take precedence over process.env.

Precendence order before:

1. Package defaults
2. env param
3. process.env

Precendence order after:

1. env param
2. Package defaults
3. process.env

Related

• Based on fix: autolocation of node-gyp in make-spawn-args #221
  • This PR goes one step further compared to base 221.
• [BUG] The npm_config_node_gyp environment variable and npm config set node-gyp config don't work with npm 7 #23
• [BUG] config var "node-gyp" not respected in npm 7 cli#2839

[wraithgar]

Thanks for your patience on this one. The reason it has sat like this is because node-gyp is not in fact valid npm config. We had planned on (and have just now started work on) making npm warn on configs it sees that aren't valid. This is so that folks are not surprised anymore when they do something like --loglvel or some other typo in any config.

This will mean finding the edges where we are still allowing for invalid configs, and formalize them in some way. node-gyp is one such config. The solution is going to be much bigger than just this PR though. We wanted to wait till work had started so we could address all the unknowns together. This PR is on the list.