Revert "Exclude extension-installer from the build"

This reverts commit 35e27f9.

Author: ondrejmirtes

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
       matrix:
         extension-name:
           - "phpstan-doctrine"
-          #- "extension-installer"
+          - "extension-installer"
           - "phpstan-phpunit"
           - "phpstan-mockery"
           - "phpstan-symfony"