[ci] Fix missing usign binary during package indexing

- Build tools/usign/compile before running make package/index
- Only build usign if binary is not already present in staging_dir/host/bin
- Fixes 'usign: No such file or directory' error in CI
- Handles both CI_CACHE and non-cache scenarios

The error occurred because make package/index requires the usign binary
to exist even when SIGNED_PACKAGES= is set to skip signing. This change
ensures usign is built when needed, particularly when CI_CACHE is set
and tools/install is skipped.

Author: shubhamk0205

runbuild

@@ -52,8 +52,12 @@ fi
 
 make -j"$CORES" package/openwisp-config/compile || make -j1 V=s package/openwisp-config/compile || exit 1
 
+# Build usign tool if not already built (required for package index generation)
+if [ ! -f staging_dir/host/bin/usign ]; then
+	make -j"$CORES" tools/usign/compile || make -j1 V=s tools/usign/compile
+fi
+
 # Generate package index with checksums (unsigned - no usign needed)
-cd "$BUILD_DIR"/openwrt
 make package/index SIGNED_PACKAGES= V=s
 
 # Filter Packages file to include only openwisp packages and save as checksum file