[StaticWebAssets] Avoid serializing/deserializing manifest for hash

[javiercn]

The actual win is around ~.5s even if it doesn't show up in this particular run. With so few measurements included (for the sake of conciseness).

Before

Build succeeded in 12.8s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.8s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.7s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.6s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.6s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.7s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.6s

After

You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.3s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.2s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.3s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.3s
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  mvc-dotnet-9 succeeded (11.3s) → bin\Debug\net9.0\mvc-dotnet-9.dll

Build succeeded in 12.2s