Remove CARGO_TARGET_CACHE_NAME, use BUILDKITE_LABEL

.buildkite/hooks/post-command

@@ -3,15 +3,14 @@
 #
 # Save target/ for the next CI build on this machine
 #
-if [[ -n $CARGO_TARGET_CACHE_NAME ]]; then
-  (
-    d=$HOME/cargo-target-cache/"$CARGO_TARGET_CACHE_NAME"
-    mkdir -p "$d"
-    set -x
-    rsync -a --delete --link-dest="$PWD" target "$d"
-    du -hs "$d"
-  )
-fi
+(
+  set -x
+  d=$HOME/cargo-target-cache/"$BUILDKITE_LABEL"
+  mkdir -p "$d"
+  set -x
+  rsync -a --delete --link-dest="$PWD" target "$d"
+  du -hs "$d"
+)
 
 #
 # Add job_stats data point

.buildkite/hooks/pre-command

@@ -11,8 +11,9 @@ export PS4="++"
 #
 # Restore target/ from the previous CI build on this machine
 #
-[[ -n "$CARGO_TARGET_CACHE_NAME" ]] || (
-  d=$HOME/cargo-target-cache/"$CARGO_TARGET_CACHE_NAME"
+(
+  set -x
+  d=$HOME/cargo-target-cache/"$BUILDKITE_LABEL"
 
   if [[ -d $d ]]; then
     du -hs "$d"
@@ -24,6 +25,5 @@ export PS4="++"
   fi
 
   mkdir -p "$d"/target
-  set -x
   rsync -a --delete --link-dest="$d" "$d"/target .
 )