fix: pin nightly version on source cov workflow (#120)

A bandaid over the issue of source coverage failing due to Dalek being a
naughty library again.

Clearly a temporary fix, since this will break again some thim in the
future.

.github/workflows/source-cov.yml

@@ -4,7 +4,7 @@ on:
  branches:
  - main
 env:
- RUSTUP_TOOLCHAIN: "nightly"
+ RUSTUP_TOOLCHAIN: "stable"
 jobs:
  coverage:
  runs-on: ubuntu-latest
@@ -18,7 +18,7 @@ jobs:
  - name: Download Rust
  uses: dtolnay/rust-toolchain@master
  with:
- toolchain: nightly
+ toolchain: {{ env.RUSTUP_TOOLCHAIN }}
  components: llvm-tools-preview
  - name: Install requirements for code coverage
  run: |