attempt at making CI faster

Move rust-nightly to stage test so it runs in parallel with coverage,
which are the two longest jobs, and currently run in sequence.
Don't document dependancies, should make the step about 50% faster
Use about 12M of cache to not recompile grcov each time
Don't compile grcov with coverage, we don't need it, it's probably
slower both to compile and execute

.gitlab-ci.yml

@@ -57,7 +57,7 @@ rust-latest-async-std-rustls:
     - amd64
 
 rust-nightly:
-  stage: build
+  stage: test
   image: rustlang/rust:nightly
   allow_failure: true
   script:
@@ -71,7 +71,7 @@ rust-nightly:
     # and similar, for when nightly clippy has false positives.  (There is no builtin
     # cfg flag for nightly, and a cargo "feature" would be bad with --all-features.)
     - cargo clippy --all-features --tests -- --cfg ci_arti_nightly -D clippy::dbg_macro -D clippy::print_stdout -D clippy::print_stderr
-    - RUSTDOCFLAGS="-Dwarnings" cargo doc --all-features --document-private-items
+    - RUSTDOCFLAGS="-Dwarnings" cargo doc --all-features --document-private-items --no-deps
   tags:
     - amd64
 
@@ -79,19 +79,20 @@ coverage:
   stage: test
   image: rust:latest
   allow_failure: true
-  variables:
-    RUSTFLAGS: "-Zinstrument-coverage"
-    LLVM_PROFILE_FILE: "arti-%p-%m.profraw"
   script:
     - apt-get update && apt-get install -y python3-pip python3-setuptools
     - pip3 install beautifulsoup4 lxml
     - rustup toolchain add nightly
     - rustup default nightly
     - rustup component add llvm-tools-preview
-    - cargo install grcov
+    - cp grcov $CARGO_HOME/bin/ || cargo install grcov
+    - cp $CARGO_HOME/bin/grcov .
     # Generate report
     - ./maint/with_coverage cargo test --verbose --all-features
     - ./maint/with_coverage -f cobertura -o coverage.xml -c true
+  cache:
+    paths:
+      - grcov
   artifacts:
     paths:
       - 'coverage'