`libafl_qemu` can not build ASAN for mips target

**IMPORTANT**
1. You have verified that the issue to be present in the current `main` branch

Thank you for making LibAFL better!

**Describe the bug**
In the `just/envs/.env.mips` file, missing relevant fields for building asan.
If we manully create them and try compile again, we will get an error because the latest Rust version does not support cross-compilation for MIPS.

**To Reproduce**
Steps to reproduce the behavior:
1. Just using `ARCH=mips just build` try to build the qemu_launcher
2. run `rustup target list | grep mips` and we can not find any mips support ....

**Expected behavior**
1. I will try to see if it's possible to compile a prebuilt version using rust cross....

**Additional context**
Add any other context about the problem here.