Error running libafl_libfuzzer_runtime/build.sh with stable toolchain

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

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**Describe the bug**
A clear and concise description of what the bug is.
If you want to present the backtrace, don't forget to run with `errors_backtrace` feature and log from `RUST_LOG`
In addition, please tell us what is your fuzzer's Cargo.toml

I'm trying to build the standalone libFuzzer compatibility library by running [`build.sh`](https://github.com/AFLplusplus/LibAFL/blob/0.15.4/crates/libafl_libfuzzer_runtime/build.sh). `nightly` is giving me build errors, so I tried to build with `stable` and get the following error:

```
#13 [libafl-builder 7/7] RUN bash build.sh --toolchain stable
#13 84.57 Finished `release` profile [optimized + debuginfo] target(s) in 1m 24s
#13 84.60 error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
#13 84.60 See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.
#13 ERROR: process "/bin/sh -c bash build.sh --toolchain stable" did not complete successfully: exit code: 101
```

**To Reproduce**
Steps to reproduce the behavior:

1. `bash build.sh --toolchain stable`

**Expected behavior**
A clear and concise description of what you expected to happen.

The library successfully builds against the stable channel.

**Screen output/Screenshots**
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**Additional context**
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