Fix IAC scan CI for new branches on Github

[Paul-GitGuardian]

First commit refactors some IAC tests to use fixtures instead of global variables, and could be reviewed separately.

Other commits fix 2 bugs for IAC and SCA:

• CI scans failed when pushing a new branch, due to CI predefined variables being incorrect -> now the parent commit is calculated using git commands
• They failed on the first push to a repository for the same reason (and because there were no commits to compare HEAD to) -> a scan all is triggered instead

Previously, for these two cases, Gitlab CI failed, while Github CI fallback was to scan the last commit only.

CI is still misbehaving on force pushes, this will be fixed in another PR.

[codecov-commenter]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (3203bfb) 91.60% compared to head (c81eb54) 91.93%. Report is 5 commits behind head on main.

Files	Patch %	Lines
ggshield/core/git_hooks/ci/previous_commit.py	65.00%	14 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #825 +/- ## ========================================== + Coverage 91.60% 91.93% +0.32% ========================================== Files 168 168 Lines 6929 6952 +23 ========================================== + Hits 6347 6391 +44 + Misses 582 561 -21 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/825/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/825/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.93% <65.00%> (+0.32%)` | :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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