Fix network settings uneditable with config command

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes a bug.

• What is the current behavior?

Calling arduino-cli config add network.proxy http://<login>:<password>@myproxy.ext:port returns Settings key doesn't exist error.

• What is the new behavior?

Calling arduino-cli config add network.proxy http://<login>:<password>@myproxy.ext:port sets correctly the network.proxy value.

• Does this PR introduce a breaking change, and is
  titled accordingly?

Nope.

• Other information:

Fixes #1273

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