Make llbuild task environment overridable from tests

[jakepetroules]

llbuild's shell tool has an inherit-env property which is true by default, and causes the process environment of the build service to be propagated to build tasks like clang. Instead we should set inherit-env to false and merge processEnvironment from UserInfo into the task's environment, so that it is overridable from tests.

This impacts the explicitModulesEnvironmentTest().