Lint against panicky calls in async functions (#3250)

## Description

Add a new lint to CI that attempts to detect calls to functions like `block_on` from async execution contexts. This lint was written from scratch exactly for this purpose, on my fork of Clippy: https://github.com/michaelsproul/rust-clippy/tree/disallow-from-async

## Additional Info

- I've successfully detected the previous two issues we had with `block_on` by running the linter on the commits prior to each of these PRs: https://github.com/sigp/lighthouse/pull/3165, https://github.com/sigp/lighthouse/pull/3199.
- The lint runs on CI with `continue-on-error: true` so that if it fails spuriously it won't block CI.
- I think it would be good to merge this PR before https://github.com/sigp/lighthouse/pull/3244 so that we can lint the extensive executor-related changes in that PR.
- I aim to upstream the lint to Clippy, at which point building a custom version of Clippy from my fork will no longer be necessary. I imagine this will take several weeks or months though, because the code is currently a bit hacky and will need some renovations to pass review.

.github/workflows/test-suite.yml

@@ -252,6 +252,23 @@ jobs:
       run: make lint
     - name: Certify Cargo.lock freshness
       run: git diff --exit-code Cargo.lock
+  disallowed-from-async-lint:
+    name: disallowed-from-async-lint
+    runs-on: ubuntu-latest
+    needs: cargo-fmt
+    continue-on-error: true
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install SigP Clippy fork
+      run: |
+        cd ..
+        git clone https://github.com/michaelsproul/rust-clippy.git
+        cd rust-clippy
+        git checkout 31a49666ccfcd7963b63345d6ce757c373f22c2a
+        cargo build --release --bin cargo-clippy --bin clippy-driver
+        cargo build --release --bin cargo-clippy --bin clippy-driver -Zunstable-options --out-dir $(rustc --print=sysroot)/bin
+    - name: Run Clippy with the disallowed-from-async lint
+      run: make nightly-lint
   check-msrv:
     name: check-msrv
     runs-on: ubuntu-latest