ci: fix CARGO_HOME caching

CARGO_HOME's semantic depends on when it's used in the pipeline:
- it's the install prefix when installing Cargo
- it's the crate cache location when building jobs

env.sh is sourced at the start of all jobs and was overidding the
CARGO_HOME path defined in the CI template for caching. Fix this by
moving the prefix install path to install-rust.sh.

.gitlab-ci.yml

@@ -35,7 +35,7 @@ stages:
 .debian:10:
   variables:
     FDO_DISTRIBUTION_VERSION: 10
-    FDO_DISTRIBUTION_TAG: '$RUST_VERSION-2020-04-16.2'
+    FDO_DISTRIBUTION_TAG: '$RUST_VERSION-2020-04-21.1'
     # Only stuff inside the repo directory can be cached
     # Override the CARGO_HOME variable to force its location
     CARGO_HOME: "${CI_PROJECT_DIR}/.cargo_home"

ci/env.sh

@@ -1,5 +1,4 @@
 export RUSTUP_HOME='/usr/local/rustup'
-export CARGO_HOME='/usr/local/cargo'
 export PATH=$PATH:/usr/local/cargo/bin
 
 export PKG_CONFIG_PATH=/usr/local/lib/x86_64-linux-gnu/pkgconfig

ci/install-rust.sh

@@ -1,4 +1,5 @@
 source ./ci/env.sh
+export CARGO_HOME='/usr/local/cargo'
 
 RUSTUP_VERSION=1.21.1
 RUST_VERSION=$1