rust-cache-save: dodge manifest templating, resolve the target against the workspace

The save input's description quoted a literal expression, which the runner template-evaluates while loading the manifest — no github context exists there, so every consumer failed to load the action.
The prune script now resolves the workspace-relative target directory against GITHUB_WORKSPACE instead of its working directory, which may sit in a nested crate for cargo metadata.

Author: gronke

.github/actions/rust-cache-save/action.yml

@@ -11,7 +11,7 @@ description: >-
 inputs:
   save:
     description: >-
-      Upload the pruned target. Pass `${{ github.ref == 'refs/heads/main' }}` for the
+      Upload the pruned target. Pass the expression "github.ref == 'refs/heads/main'" for the
       restore-everywhere/save-on-main pattern.
     required: false
     default: "true"

.github/actions/rust-cache-save/prune-target.sh

@@ -7,7 +7,14 @@
 # DRY_RUN=1 prints what would go instead of removing it, for local runs.
 set -euo pipefail
 
+# RUST_CI_TARGET_DIR is workspace-relative (the same string the cache paths
+# use), while this script may run in a nested working-directory for cargo
+# metadata — resolve against the workspace, not the cwd.
 target="${RUST_CI_TARGET_DIR:?rust-cache did not export RUST_CI_TARGET_DIR}"
+case "$target" in
+  /*) ;;
+  *) target="${GITHUB_WORKSPACE:-$PWD}/$target" ;;
+esac
 if [ ! -d "$target" ]; then
   echo "no $target directory; nothing to prune"
   exit 0