cli: use cached index with lib search (#1624)

[ardnew]

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Please check if the PR fulfills these requirements

• [x] The PR has no duplicates (please search among the Pull Requests before creating one)
• [x] 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? Feature

• What is the current behavior? A new copy of library_index.json is downloaded every time lib search is called (unconditionally).

• What is the new behavior? A new copy of library_index.json is downloaded every time lib search is called and either one of the following conditions exists:
  1. The index has never been downloaded
  2. The time since the last index update is greater than one hour

• Does this PR introduce a breaking change, and is titled accordingly? No

• Other information:
  • Fixes: #1624
  • Changes based on: #1063
  • Need assistance adding test cases and documentation for this change

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[cmaglie]

I think that just removing these two lines https://github.com/arduino/arduino-cli/blob/f556aff7079ac5b02683f305329c90f8755c50f0/test/test_lib.py#L456-L457 should be enough to fix the integration tests.