Bump actix-rt from 2.7.0 to 2.8.0 (#577)

Bumps [actix-rt](https://github.com/actix/actix-net) from 2.7.0 to 2.8.0.
- [Release notes](https://github.com/actix/actix-net/releases)
- [Commits](actix/actix-net@rt-v2.7.0...rt-v2.8.0)

---
updated-dependencies:
- dependency-name: actix-rt
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

crates/backend/Cargo.toml

@@ -23,5 +23,5 @@ tokio-compat-02 = "0.2.0"
 sandbox = { path = "../sandbox" }
 
 [dev-dependencies]
-actix-rt = "2.7.0"
+actix-rt = "2.8.0"