minor #2780 [CI] Replace ramsey/composer-install by composer install in app-tests workflow (Kocal)

This PR was merged into the 2.x branch.

Discussion
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[CI] Replace ramsey/composer-install by `composer install` in app-tests workflow

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Docs?         | no <!-- required for new features -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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In #2607, https://github.com/symfony/ux/actions/runs/15297531864/job/43030093028?pr=2607 is shown as failed because step `Run Encore (dev)`, but in reality it fails before but that's not shown:
![image](https://github.com/user-attachments/assets/70d2d886-63c0-4147-a4e7-91447a5870a9)

With this change (which also remove a dependency), I hope the error to be well shown

Commits
-------

5db7b99 [CI] Replace ramsey/composer-install by `composer install` in app-tests workflow

Author: Kocal

.github/workflows/app-tests.yaml

@@ -45,19 +45,15 @@ jobs:
         - uses: shivammathur/setup-php@v2
 
         - name: Install root dependencies
-          uses: ramsey/composer-install@v3
-          with:
-            working-directory: ${{ github.workspace }}
+          run: composer install
 
         - name: Build root packages
           run: php .github/build-packages.php
-          working-directory: ${{ github.workspace }}
 
         # We always install PHP deps because we of the UX Translator, which requires `var/translations` to exists
-        - uses: ramsey/composer-install@v3
-          with:
-            dependency-versions: 'highest'
-            working-directory: test_apps/encore-app
+        - name: Install App dependenies
+          run: composer update
+          working-directory: test_apps/encore-app
 
         - if: matrix.ux-packages-source == 'php-vendor'
           name: Refresh dependencies from vendor/