Fix ruby cache key calculation used for npm packaging

The `RUBY_WASM_ROOT` environment variable is not set when calculating
ruby cache keys for npm packages and rbwasm assumes that the
build_manifest.json file is placed in the cwd (npm package directory).
But the build_manifest.json file is placed in the repository root
so the cache key calculation didn't respect the pinned ruby revision.

Rakefile

@@ -68,8 +68,9 @@ class BuildTask < Struct.new(:name, :target, :build_command)
   def ruby_cache_key
     return @key if @key
     require "open3"
+    env = { "RUBY_WASM_ROOT" => LIB_ROOT }
     cmd = build_command + ["--print-ruby-cache-key"]
-    stdout, status = Open3.capture2(*cmd)
+    stdout, status = Open3.capture2(env, *cmd)
     unless status.success?
       raise "Command failed with status (#{status.exitstatus}): #{cmd.join " "}"
     end

rakelib/packaging.rake

@@ -30,7 +30,8 @@ def npm_pkg_rubies_cache_key(pkg)
   require "open3"
   cmd = build_command + ["--print-ruby-cache-key"]
   chdir = pkg[:gemfile] ? File.dirname(pkg[:gemfile]) : Dir.pwd
-  stdout, status = Open3.capture2(*cmd, chdir: chdir)
+  env = { "RUBY_WASM_ROOT" => LIB_ROOT }
+  stdout, status = Open3.capture2(env, *cmd, chdir: chdir)
   unless status.success?
     raise "Command failed with status (#{status.exitstatus}): #{cmd.join " "}"
   end