Use stable image and download nightly in coverage

Following suggestion here:
`tpo/core/arti#250 (comment 2764010)`

.gitlab-ci.yml

@@ -52,11 +52,13 @@ rust-nightly:
 
 coverage:
   stage: test
-  image: rustlang/rust:nightly
+  image: rust:latest
   variables:
     RUSTFLAGS: "-Zinstrument-coverage"
     LLVM_PROFILE_FILE: "coverage-%p-%m.profraw"
   script:
+    - rustup toolchain add nightly
+    - rustup default nightly
     - rustup component add llvm-tools-preview
     - cargo build --verbose --target x86_64-unknown-linux-gnu --all-features
     - cargo test --verbose --target x86_64-unknown-linux-gnu --all-features