fix(ci): harden cargo build retry by wiping target dir and disabling sccache (#77)

The multi-arch publish job fails with 'extern location does not exist'
errors for version_check when BuildKit cache mounts contain stale
artifacts. The existing retry logic (cargo clean + retry) was
ineffective because:

1. cargo clean relies on its own metadata to decide what to remove,
   which can itself be stale on BuildKit cache mounts shared across
   different build profiles (debug vs release).

2. sccache remained enabled during the retry, so a corrupt memcached
   cache entry would reproduce the same failure even after cleaning.

Replace cargo clean with rm -rf /build/target/* for a thorough wipe,
and unset RUSTC_WRAPPER on retry so cargo falls back to raw rustc.

Author: drew

deploy/docker/cross-build.sh

@@ -108,13 +108,17 @@ cargo_cross_build() {
   # "No such file or directory" writing .d files if deps/ is missing.
   mkdir -p "$(cross_output_dir "$profile")/deps"
   # Retry once after cleaning if the build fails. BuildKit cargo-target cache
-  # mounts can retain stale .rmeta files from prior builds with different
-  # dependency versions; cargo clean purges them so the retry succeeds.
-  # sccache still has the compiled objects, so the clean rebuild is fast.
+  # mounts can retain stale .rmeta/.rlib files from prior builds with different
+  # dependency versions or profiles. We wipe the entire target directory
+  # (rm -rf is more thorough than cargo clean, which relies on its own stale
+  # metadata) and disable sccache for the retry — a corrupt sccache cache
+  # entry can cause "extern location does not exist" errors even on a
+  # freshly-cleaned target dir, so falling back to raw rustc is safer.
   if ! cargo build $target_flag "$@"; then
-    echo "cargo build failed; cleaning stale target cache and retrying..." >&2
-    cargo clean 2>/dev/null || true
+    echo "cargo build failed; cleaning stale target cache and retrying without sccache..." >&2
+    rm -rf /build/target/*
     mkdir -p "$(cross_output_dir "$profile")/deps"
+    unset RUSTC_WRAPPER 2>/dev/null || true
     cargo build $target_flag "$@"
   fi
 }