fix(CI): bump rust version, mdbook-linkcheck: fix path+caching

.gitlab-ci.yml

@@ -18,7 +18,7 @@ format:
     - cargo +nightly fmt --all -- --check
 
 linux:
-  image: 'rust:1.39.0'
+  image: 'rust:1.43.0'
   cache:
     paths:
       - cargo/
@@ -49,11 +49,16 @@ redox:
 
 link-check:
   image: hrektts/mdbook
+  cache:
+    paths:
+      - cargo/
+      - cargo/bin
   before_script:
     - apt-get update -qq
     - apt-get install -qq libssl-dev pkg-config build-essential curl git
-    - (test -x $HOME/.cargo/bin/mdbook-linkcheck || cargo install mdbook-linkcheck)
+    - test -x $CARGO_HOME/bin/mdbook-linkcheck || cargo install mdbook-linkcheck
   script:
+  - PATH=$PATH:$CARGO_HOME/bin
   - make manual
   - mdbook build manual