Change Build.fs to explicitly restore

[isaacabraham]

In light of #621 I think that we should change the run process:

1. Do a standard dotnet restore
2. Do a dotnet watch run --no-restore

This has two benefits:

1. It solves this weird issue that some people are seeing with watch run.
2. It will give a small perf improvement to watch reloads because the restore process is skipped on restarts.

[jwthomson]

I am looking into this

[Larocceau]

closed by #634

[isaacabraham]

I'm reopening this because of my comments in #634 and assigning it to @Larocceau .