Install llvm-cov on nightly to revive coverage

Towards solana-labs#433

ci/docker-rust-nightly/Dockerfile

@@ -1,3 +1,4 @@
 FROM rustlang/rust:nightly
 
 RUN cargo install --force clippy cargo-cov
+RUN apt update && apt-get install -y llvm-6.0

ci/test-nightly.sh

@@ -15,9 +15,6 @@ _ cargo build --verbose --features unstable
 _ cargo test --verbose --features unstable
 _ cargo clippy -- --deny=warnings
 
-exit 0
-
-# Coverage disabled (see issue #433)
 _ cargo cov test
 _ cargo cov report
 
@@ -27,6 +24,6 @@ ls -l target/cov/report/index.html
 if [[ -z "$CODECOV_TOKEN" ]]; then
   echo CODECOV_TOKEN undefined
 else
-  bash <(curl -s https://codecov.io/bash) -x 'llvm-cov gcov'
+  bash <(curl -s https://codecov.io/bash) -x 'llvm-cov-6.0 gcov'
 fi