Add cpyext tests build directory to .gitignore

Author: steve-s

.gitignore

@@ -90,5 +90,6 @@ graalpython/graalpy-maven-plugin/target
 graalpython/graalpy-archetype-polyglot-app/target
 graalpython/graalpy-micronaut-embedding/target
 graalpython/com.oracle.graal.python.test/src/tests/standalone/micronaut/hello/target/
+graalpython/com.oracle.graal.python.test/src/tests/cpyext/build/
 pom-mx.xml
-.venv
+.venv