Unnecessary rebuilds caused because`GenerateNuSpec` writes out the file even if the contents are same

I tried this with a local build of `dotnet/runtime` on macOS, with:

`$ ./build.sh mono+libs -os Browser -c Release`

In the beginning of the build you will see a line:

`Microsoft.NET.Runtime.wasm.Sample.Mono -> /Users/radical/dev/r3/artifacts/packages/Release/Shipping/Microsoft.NET.Runtime.wasm.Sample.Mono.6.0.0-dev.nupkg`

The nuspec this used: `artifacts/packages/Release/specs/Microsoft.NET.Runtime.wasm.Sample.Mono.nuspec`

Then if I build again, then this file gets generated again, and written out, thus updating the timestamp.

The files are identical, AFAICS:

```
$ shasum /tmp/sample.nuspec
d353e675930dac28fa0d52637593e1adf066c325 /tmp/sample.nuspec

$ shasum /Users/radical/dev/r3/artifacts/packages/Release/specs/Microsoft.NET.Runtime.wasm.Sample.Mono.nuspec
d353e675930dac28fa0d52637593e1adf066c325 /Users/radical/dev/r3/artifacts/packages/Release/specs/Microsoft.NET.Runtime.wasm.Sample.Mono.nuspec
```

This causes the nupkg to get rebuilt every time, which can cascade into other things getting rebuilt.