minor symfony#28342 [appveyor] fix bis (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[appveyor] fix bis

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Give it another try...

Commits
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9d24400 [appveyor] fix bis

Author: nicolas-grekas

appveyor.yml

@@ -56,7 +56,7 @@ install:
     - php composer.phar self-update
     - copy /Y .composer\* %APPDATA%\Composer\
     - php composer.phar global require --no-progress --no-scripts --no-plugins symfony/flex dev-master
-    - php .github/build-packages.php "HEAD^" src\Symfony\Bridge\PhpUnit
+    - php .github/build-packages.php %APPVEYOR_REPO_COMMIT%^^ src\Symfony\Bridge\PhpUnit
     - IF %APPVEYOR_REPO_BRANCH%==master (SET COMPOSER_ROOT_VERSION=dev-master) ELSE (SET COMPOSER_ROOT_VERSION=%APPVEYOR_REPO_BRANCH%.x-dev)
     - php composer.phar config platform.php 5.3.9
     - php composer.phar update --no-progress --no-suggest --ansi