Better hash checks in bootstrapper

[vbfox]

Quite a few changes related to hash file handling in the bootstrapper

• Correctly cache the hash file (It was never cashed before even if the cache strategy expected it to be)
• Make hash downloading a top-level operation only done when we know that the local version isn't the one we want
  • Avoid downloading it only to check the remote version
  • Also avoid downloading twice when TemporarilyIgnoreUpdatesDownloadStrategy is used
• Check hash after every download from GitHub
  • It was only used to check if our cache is valid before, it's now being used to check if the download is valid directly after doing it
  • We re-try once if the hash is invalid and only warn if still invalid after a second download (Can be changed later, maybe we should let the fallback happen and download from nuget ?)

Fixes #2365

I tested manually and don't see any weird behavior anymore with useless downloads. I'll add some unit tests to confirm and the PR will be good to go

[matthid]

Wonderful, thanks again for taking care of this!

[matthid]

@vbfox I tried to add the bootstrapper again but I got:

C:\projects\fake\.paket\Paket.Restore.targets(17,5): error MSB3073: The command ""C:\projects\fake\.paket\paket.exe" restore --project "C:\projects\fake\src\app\Fake.Core.Targets\Fake.Core.Targets.fsproj" --target-framework netstandard1.6" exited with code 1. [C:\projects\fake\src\app\Fake.Core.Targets\Fake.Core.Targets.fsproj]
  Could not find a part of the path 'C:\Users\appveyor\AppData\Local\NuGet\Cache\Paket\5.0.0-beta006\paket-sha256.txt'. (Github - cached)

on Appveyor

and Travis

/home/travis/build/fsharp/FAKE/.paket/Paket.Restore.targets(17,5): error MSB3073: The command "mono --runtime=v4.0.30319 "/home/travis/build/fsharp/FAKE/.paket/paket.exe" restore --project "/home/travis/build/fsharp/FAKE/src/app/Fake.Core.BuildServer/Fake.Core.BuildServer.fsproj" --target-framework netstandard1.6" exited with code 1. [/home/travis/build/fsharp/FAKE/src/app/Fake.Core.BuildServer/Fake.Core.BuildServer.fsproj]

  Destination directory not found: /home/travis/.local/share/NuGet/Cache/Paket/5.0.0-beta006 (Github - cached)

Maybe I did something wrong?

[vbfox]

I suspect I just fucked up by not testing the case where the cache folder doesn't exists at all. I'll fix it