[BUG] Cannot set _auth without first setting email

Current Behavior:

npm config set _auth foo
# Error: Cannot set _auth without first setting email 

npm config set email foo@example.com

npm config get email # undefined ?????

So I try adding email to my npmrc directly, then:

npm config get email # foo@example.com

npm config set proxy http://localhost:3128
npm config get email # undefined ??!?!?!?

Seems like doing npm config set gets rid of your email, no matter which key you are setting (including email itself)

Expected Behavior:

• npm config set to not delete email setting
• npm config set _auth to work regardless of whether email is set.

Steps To Reproduce:

1. Upgrade to latest npm 7
2. Run npm set config _auth foo
3. See error...

Environment:

• OS: MacOS Catalina 10.15.7
• Node: 14.13.0
• npm: 7.6.2

Log:

20 info config set "_auth" "foo"
21 timing command:config Completed in 1ms
22 verbose stack Error: Cannot set _auth without first setting email
22 verbose stack     at Config.set (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/node_modules/@npmcli/config/lib/index.js:184:15)
22 verbose stack     at Config.set (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/config.js:123:23)
22 verbose stack     at Config.config (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/config.js:91:22)
22 verbose stack     at Config.exec (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/config.js:83:10)
22 verbose stack     at Object.[_runCmd] (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/npm.js:112:12)
22 verbose stack     at /usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/npm.js:28:35
22 verbose stack     at Object.<anonymous> (/usr/local/Cellar/node/14.13.0/lib/node_modules/npm/lib/cli.js:58:7)

Start with @npmcli/config/lib/index.js at Config.set and follow the calls through npm/lib/config.js. Reproduce the npm 7 commands shown, then verify that setting a key preserves email and that setting _auth works without requiring email.