ci: fix rust emulator build failure

```
error: "/home/runner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/Cargo.lock" does not exist, unable to build with the standard library, try:
        rustup component add rust-src --toolchain nightly-x86_64-unknown-linux-gnu
```

https://github.com/bitcoin-core/HWI/actions/runs/16440824447/job/46462763374?pr=795

Author: Sjors

test/setup_environment.sh

@@ -110,6 +110,7 @@ if [[ -n ${build_trezor_1} || -n ${build_trezor_t} ]]; then
         rustup toolchain install nightly
         rustup default nightly
         rustup component add rustfmt
+        rustup component add rust-src --toolchain nightly-x86_64-unknown-linux-gnu
         # Build trezor t emulator. This is pretty fast, so rebuilding every time is ok
         # But there should be some caching that makes this faster
         poetry install