gitlab-ci: use a stable build directory

This should allow for sccache to get better hit rates with its builds.

.gitlab-ci.yml

@@ -69,6 +69,7 @@ before_script:
     variables:
         CARGO_UPDATE_POLICY: newest
         SCCACHE_REDIS: redis://minmus:6379
+        GIT_CLONE_PATH: $CI_BUILDS_DIR/utils/ghostflow-director
 
 .rust_stable: &rust_stable
     image: "rust:latest"
@@ -76,6 +77,7 @@ before_script:
     variables:
         CARGO_UPDATE_POLICY: newest
         SCCACHE_REDIS: redis://minmus:6379
+        GIT_CLONE_PATH: $CI_BUILDS_DIR/utils/ghostflow-director
 
 .rust_nightly: &rust_nightly
     extends: .rust_stable
@@ -347,6 +349,7 @@ prepare:git:
         - popd
     variables:
         GIT_STRATEGY: none
+        GIT_CLONE_PATH: $CI_BUILDS_DIR/utils/rust-git-workarea
     artifacts:
         expire_in: 60m
         paths: