limit ignore-platform-reqs to php only for php 8

Author: dbu

.travis.yml

@@ -41,7 +41,7 @@ matrix:
     - php: 7.4
       env: SYMFONY_REQUIRE=5.0.*
     - php: nightly
-      env: COMPOSER_FLAGS="--no-interaction --ignore-platform-reqs" SYMFONY_PHPUNIT_VERSION=9 SYMFONY_REQUIRE=5.0.*
+      env: COMPOSER_FLAGS="--no-interaction --ignore-platform-req=php" SYMFONY_PHPUNIT_VERSION=9 SYMFONY_REQUIRE=5.0.*
   fast_finish: true
   allow_failures: