Allow omnibus to disable sourcemaps in memory constrained environments

Some memory-limited environments like Raspberry Pi cannot handle the webpack compile process. Allowing it to disable sourcemaps for certain builds has the effect of reducing the memory requirement by around 35-45%.

This has been tested with a hard-coded value and confirmed to fix our failing RasPi builds:
https://dev.gitlab.org/gitlab/omnibus-gitlab/-/jobs/2552517

Related: #47656 (moved)