Add shellcheck CI (#7888)

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## Summary

I started learning `uv` by inspecting the source code.
I've noticed that your shell scripts are very good! Which is rare!

## Test Plan

I propose to add `shellcheck` to the CI.
It is a great tool to help finding bugs and style issues in shell code.

Techincal details:
- This CI job will only run when any `.sh` files are changed (or the job
definition file)
- It takes just several seconds even on local machine:
```
» time shellcheck -S style **/*.sh
shellcheck -S style **/*.sh  0.02s user 0.05s system 61% cpu 0.123 total
```

- It is easy to use, for example: I just fixed the single problem you
had in your code with `# shellcheck disable=SC1091`
- I am using this tool for around 8 years now and didn't have any
issues. Examples:
ca899f3b69/.github/workflows/test.yml (L22-L27)
and
https://github.com/wemake-services/wemake-django-template/blob/master/.github/workflows/shellcheck.yml

But, I understand that build / lint tools are very subjective. So, feel
free to close :)

scripts/sync_scenarios.sh

@@ -25,6 +25,7 @@ cd "$script_root/scenarios"
 echo "Setting up a temporary environment..."
 uv venv
 
+# shellcheck disable=SC1091
 source ".venv/bin/activate"
 uv pip install -r requirements.txt --refresh-package packse