With the new flag, Starlark actions that specify both `env` and `use_default_shell_env` will no longer have `env` ignored. Instead, the values of `env` will override the default shell environment.

This allows Starlark actions to both pick up user-configured variables such as `PATH` from the shell environment as well as set variables to fixed values required for the action, e.g., variables provided by the C++ toolchain.

Rationale for having `env` override the default shell env: The rules know best which values they have to set specific environment variables to in order to successfully execute an action, so a situation where users break an action by a globally applied `--action_env` is prevented. If users really do have to be able to modify an environment variables fixed by the rule, the rule can always make this configurable via an attribute.

Work towards bazelbuild#5980
Fixes bazelbuild#12049

Closes bazelbuild#18235.

PiperOrigin-RevId: 559506535
Change-Id: I7ec6ae17b076bbca72fab8394f3a8b3e4f9ea9d8

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