Fix release workflow

We temporarily need to add the `--cfg tokio_unstable` flag. Once we
release the scheduler for Linux, this won't be needed anymore in the
release script. Instead of building the scheduler there to query the
version, we will use the already built binary.

Author: jherbel

.github/workflows/release.yaml

@@ -37,7 +37,7 @@ jobs:
       - name: "Compute release tag"
         id: compute-tag
         run: |
-            version="$(cargo run --bin robotmk_scheduler -- --version | cut --delimiter " " --fields 2)"
+            version="$(RUSTFLAGS="--cfg tokio_unstable" cargo run --bin robotmk_scheduler -- --version | cut --delimiter " " --fields 2)"
             echo "TAG=v${version}" >> "${GITHUB_OUTPUT}"
 
       - name: "Push release tag"