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 :)

.github/renovate.json5

@@ -2,7 +2,11 @@
   $schema: "https://docs.renovatebot.com/renovate-schema.json",
   dependencyDashboard: true,
   suppressNotifications: ["prEditedNotification"],
-  extends: ["config:recommended"],
+  extends: [
+    "config:recommended",
+    // For tool versions defined in GitHub Actions:
+    "customManagers:githubActionsVersions",
+  ],
   labels: ["internal"],
   schedule: ["before 4am on Monday"],
   semanticCommits: "disabled",

.github/workflows/ci.yml

@@ -81,6 +81,17 @@ jobs:
       - name: "Python type check"
         run: uvx mypy
 
+      - name: "Lint shell scripts"
+        uses: ludeeus/action-shellcheck@2.0.0
+        env:
+          # renovate: datasource=github-tags depName=koalaman/shellcheck
+          SHELLCHECK_VERSION: "v0.10.0"
+          SHELLCHECK_OPTS: --shell bash
+        with:
+          version: ${{ env.SHELLCHECK_VERSION }}
+          severity: style
+          check_together: "yes"
+
   cargo-clippy:
     timeout-minutes: 10
     needs: determine_changes

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