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.

Makefile

@@ -12,6 +12,7 @@ AARCH64_TAG = "aarch64-unknown-linux-gnu"
 BUILD_PATH_AARCH64 = "target/$(AARCH64_TAG)/release"
 
 PINNED_NIGHTLY ?= nightly
+CLIPPY_PINNED_NIGHTLY=nightly-2022-05-19
 
 # List of all hard forks. This list is used to set env variables for several tests so that
 # they run for different forks.
@@ -145,6 +146,14 @@ lint:
 		-A clippy::upper-case-acronyms \
 		-A clippy::vec-init-then-push
 
+# FIXME: fails if --release is added due to broken HTTP API tests
+nightly-lint:
+	cp .github/custom/clippy.toml .
+	cargo +$(CLIPPY_PINNED_NIGHTLY) clippy --workspace --tests -- \
+		-A clippy::all \
+		-D clippy::disallowed_from_async
+	rm clippy.toml
+
 # Runs the makefile in the `ef_tests` repo.
 #
 # May download and extract an archive of test vectors from the ethereum