respect LIB_WASI_VFS_A if defined

Author: kateinoigakukun

lib/ruby_wasm/build_system/product/crossruby.rb

@@ -167,7 +167,8 @@ def with_zlib(zlib)
 
     def with_wasi_vfs(wasi_vfs)
       @wasi_vfs = wasi_vfs
-      @dep_tasks << wasi_vfs.install_task
+      install_task = wasi_vfs.install_task
+      @dep_tasks << install_task if install_task
     end
 
     def dest_dir

lib/ruby_wasm/build_system/product/wasi_vfs.rb

@@ -20,14 +20,15 @@ def lib_product_build_dir
     end
 
     def lib_wasi_vfs_a
-      File.join(lib_product_build_dir, "libwasi_vfs.a")
+      ENV["LIB_WASI_VFS_A"] || File.join(lib_product_build_dir, "libwasi_vfs.a")
     end
 
     def name
       lib_product_build_dir
     end
 
     def define_task
+      return if ENV["LIB_WASI_VFS_A"]
       @install_task =
         file(lib_wasi_vfs_a) do
           require "tmpdir"