construct_arguments: Always use correct attributes in an aspect context. (#668)

`construct_arguments` was already using the `rule_attrs` helper function
in some places to access `ctx.rule.attr` instead of `ctx.attr`, but it
wasn't doing this consistently.

I've chosen to eliminate the boolean `aspect` parameter of
`construct_arguments` and pass in the correct attributes directly
instead. This also eliminates the need for the `rule_attrs` helper
function. Since this function was only being used here, I have
eliminated it entirely.

Finally, I have changed `_get_rustc_env` to take only the attributes
instead of the entire context.

All tests continue to pass. I haven't added any tests that would break
without this change, but in an upcoming PR, I will change
`_get_rustc_env` to respect the `crate_name` attribute, and this would
break without the changes in this PR.