fix: do not automatically extend eslint configuration

Since ESLint changed their plugin resolution mechanisms for ESLint 6 and
after upgrading to ESLint 6 in cli-style 7, this changes fixes the
problem where `cli-app-scripts build` fails for apps that use
cli-style.

Since we manually extend the existing ESLint configuration from the
project in the shell ESLint configuration, and all dependencies are
hoisted, we can avoid leveraging ESLint's internal module resolution
which is based on CWD (project/.d2/shell).

shell/.env

@@ -1,2 +1,2 @@
 SKIP_PREFLIGHT_CHECK=true
-EXTEND_ESLINT=true
+EXTEND_ESLINT=false