build/bin/sage-spkg: Add missing 'cd'

Author: Matthias Koeppe

build/bin/sage-spkg

@@ -594,6 +594,7 @@ cd "\$SAGE_PKG_DIR" 2>/dev/null || cd "$fallback_script_dir"
 __EOF__
     else
         cat >> "$tmpscript" <<__EOF__
+cd "\$SAGE_PKG_DIR"
 if [ \$? -ne 0 ]; then
     echo >&2 "Error: could not cd to the package build directory \$SAGE_PKG_DIR"
     exit 1