[BUG] npm config set unpacks _auth into username/password

Is there an existing issue for this?

• I have searched the existing issues
• relates to #2300

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

npm config set registry <url> messed up pre-existing ~/.npmrc containing _auth

$ cat ~/.npmrc
//registry.npmjs.org/:username=user
//registry.npmjs.org/:_password="cGFzcw=="
registry=https://npm.example.com/

Expected Behavior

$ cat ~/.npmrc
_auth=dXNlcjpwYXNz
registry=https://npm.example.com/

Steps To Reproduce

1. Using the latest official node container image:

   docker run -it --rm docker.io/library/node:17.9.0-alpine ash
   

2. Update to the latest npm

   npm install -g npm@8.7.0
   

3. Having a existing ~/.npmrc containing _auth:

   cat << EOF > ~/.npmrc
   _auth=$(echo -n 'user:pass' | base64)
   EOF
   

4. Set the container registry:

   npm config set registry https://npm.example.com
   

5. Now the .npmrc is messed up cat ~/.npmrc:

   //registry.npmjs.org/:username=user
   //registry.npmjs.org/:_password="cGFzcw=="
   registry=https://npm.example.com/
   

Environment

• npm: v8.5.0 v8.7.0
• Node.js: v16.14.2 v17.9.0
• OS Name: Alpine Linux v3.11 v3.15
• System Model Name: Linux (in VM)
• npm config: (empty before reproduction)

Reproduce the issue with the provided Node 17 Alpine container, npm 8.7.0, and the sample ~/.npmrc, then trace the npm config set registry entry point and its handling of _auth credentials. Done means updating the registry no longer unpacks _auth into username and password, while preserving the expected .npmrc content.