ci: Intermodal command needs rust nigthly

Coverage workflow is broken becuase Intermodal console app needs the
Rust nigthly toolchain.

```
Run cargo install imdl
    Updating crates.io index
 Downloading crates ...
  Downloaded imdl v0.1.13
  Installing imdl v0.1.13
error: failed to compile `imdl v0.1.13`, intermediate artifacts can be found at `/tmp/cargo-installB9h5OI`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.

Caused by:
  failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/runner/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names -Z profile -C codegen-units=1 -C inline-threshold=0 -C link-dead-code -C overflow-checks=off -C panic=abort -Z panic_abort_tests --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 1)
  --- stderr
  error: the option `Z` is only accepted on the nightly compiler

  help: consider switching to a nightly toolchain: `rustup default nightly`

  note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

  note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>

  error: 1 nightly option were parsed

Error: Process completed with exit code 101.
```

.github/workflows/coverage.yaml

@@ -38,7 +38,7 @@ jobs:
         name: Setup Toolchain
         uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: stable
+          toolchain: nightly
           components: llvm-tools-preview
 
       - id: cache