fix(generator): add --no-build-isolation when installing local google-api-core in showcase sessions

Author: hebaalazzeh

packages/gapic-generator/noxfile.py

@@ -392,7 +392,7 @@ def showcase_library(
             # Install the library without a constraints file.
             session.install("-e", tmp_dir)
 
-        session.install("-e", "../google-api-core", "--no-deps")
+        session.install("-e", "../google-api-core", "--no-build-isolation", "--no-deps")
 
         yield tmp_dir