fix cache & snapcraft CI build (#8052)

after successful testing it is necessary to port in a ```beta``` and ```stable```

.gitlab-ci.yml

@@ -8,12 +8,10 @@ variables:
   CARGOFLAGS: ""
   CI_SERVER_NAME: "GitLab CI"
   LIBSSL: "libssl1.0.0 (>=1.0.0)"
-  CARGO_HOME: $CI_PROJECT_DIR/cargo
 cache:
   key: "$CI_BUILD_STAGE-$CI_BUILD_REF_NAME"
   paths:
     - target/
-    - cargo/
   untracked: true
 linux-stable:
   stage: build
@@ -132,7 +130,7 @@ linux-aarch64:
     name: "aarch64-unknown-linux-gnu_parity"
 linux-snap:
   stage: build
-  image: snapcore/snapcraft:stable
+  image: parity/snapcraft:gitlab-ci
   only:
     - stable
     - beta