build(just): use --skip-if-exists and surface push errors

`--skip-if-identical` was too strict for non-reproducible wasm
builds — every CI re-run produces a different layer digest at the
same source version, so the strict policy bricks every CI-only
push. Switch to --skip-if-exists (silent skip when a tag already
exists, regardless of content). Also restructure the OUTPUT
capture to echo on failure — set -e + $() previously swallowed
the actbuild error message.

Author: GamePad64

justfile

@@ -37,9 +37,9 @@ publish:
     OUTPUT=$({{actbuild}} push {{wasm}} "{{registry}}/$NAME:$VERSION" \
       --skip-if-exists \
       --also-tag latest \
-      --source "$SOURCE" 2>&1)
+      --source "$SOURCE" 2>&1) || { echo "$OUTPUT" >&2; exit 1; }
     echo "$OUTPUT"
-    DIGEST=$(echo "$OUTPUT" | grep "^Digest:" | awk '{print $2}')
+    DIGEST=$(echo "$OUTPUT" | grep "^Digest:" | awk '{print $2}' || true)
     if [ -n "${GITHUB_OUTPUT:-}" ]; then
       echo "image={{registry}}/$NAME" >> "$GITHUB_OUTPUT"
       echo "digest=$DIGEST" >> "$GITHUB_OUTPUT"