Allow controlling rustc stacker segment sizes via the environment?

[sunshowers]

While attempting to diagnose #128422, one of the issues that came up was that it is impossible to control stacker's segment sizes via the environment.

Typically, stack sizes can be controlled via the RUST_MIN_STACK environment variable, but that doesn't work for the stacker-based stacks that rustc creates. The code that invokes stacker is here, and it currently hardcodes a 1MiB stack size. The ability to set a larger stack size would have made it possible to work around the issue in #128422.

Would it make sense to have an environment variable for this? There are a few possible directions:

1. If RUST_MIN_STACK is set, create stack segments with size max(1MiB, RUST_MIN_STACK). I think generally speaking, if RUST_MIN_STACK is set in the environment, there's probably some kind of stack exhaustion that's being worked around. This is the most obvious way to do it I think.
2. Have a separate environment variable. The environment variable doesn't have to be documented -- even if it just exists in source code, enterprising folks will be able to find it.

I'm not sure if this should be filed here or in https://github.com/rust-lang/stacker/, but I think it's worth addressing somewhere.

[sunshowers]

Ah, I think this is better discussed on irlo: https://internals.rust-lang.org/t/allow-controlling-rustc-stacker-segment-sizes-via-the-environment/21292