fix: Fix ignored paths in GGShield

[gg-jonathangriffe]

This MR :

• Adds a fixture in auto use to clear the cache from functools.lru_cache between tests
• Makes ggshield get the local config from the root of the repo (if it can find git and is scanning a git repo)
• Corrects the check to return an error when scanning an ignored directory
• Fixes the test test_retrieve_client_invalid_api_url

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.81%. Comparing base (30afb5e) to head (f760485). Report is 6 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #857 +/- ## ========================================== + Coverage 91.76% 91.81% +0.05% ========================================== Files 170 170 Lines 7058 7066 +8 ========================================== + Hits 6477 6488 +11 + Misses 581 578 -3 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/857/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/857/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.81% <100.00%> (+0.05%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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