[landing] fixed fatal bundling issue using window outside function scope

Summary:
after merging D4510, for some reason, there is an issue bundling with using the global variable window in the global scope as opposed to inside a function scope. it results in this error message:

Could not load landing.build.cjs. Did you forget to run `yarn dev` in the landing folder?
at getWebpackCompiledRootComponentForSSR (file:///Users/derek/clients/comm/keyserver/dist/responders/landing-handler.js:84:11)

Test Plan: run yarn dev and be able to visit /landing instead of being greeted by a fatal error

Reviewers: ashoat, atul, abosh

Reviewed By: ashoat

Subscribers: tomek, abosh, atul, ashoat, jacek

Differential Revision: https://phab.comm.dev/D4960