[Xamarin.Build.Download] Guard against race condition when multiple processes are downloading same artifact.

[jpobst]

Occasionally on CI we see messages like this:

/Users/runner/.nuget/packages/xamarin.build.download/0.11.3/buildTransitive/Xamarin.Build.Download.targets(52,3): 
The file '/Users/runner/Library/Caches/XamarinBuildDownload/playservicesbasement-18.1.0/playservicesbasement-18.1.0.aar'
already exists. [/Users/runner/work/1/s/generated/com.google.assistant.appactions.widgets/com.google.assistant.appactions.widgets.csproj]

In the log we can see that we tried to use XDB to download this file twice at the same time:

// com.google.assistant.appactions.suggestions.csproj
Start: 2022-10-12 16:00:34.7892260
End: 2022-10-12 16:00:35.4315150
Downloading https://dl.google.com/dl/android/maven2/com/google/android/gms/play-services-basement/18.1.0/play-services-basement-18.1.0.aar to /Users/runner/Library/Caches/XamarinBuildDownload/playservicesbasement-18.1.0.aar

// com.google.assistant.appactions.widgets.csproj
Start: 2022-10-12 16:00:35.1521780
End: 2022-10-12 16:00:35.7901080
Downloading https://dl.google.com/dl/android/maven2/com/google/android/gms/play-services-basement/18.1.0/play-services-basement-18.1.0.aar to /Users/runner/Library/Caches/XamarinBuildDownload/playservicesbasement-18.1.0.aar

The first one succeeds but the second one fails with the "file already exists" errors.

Attempt to guard against this by checking for the "already downloaded" flag a second time once we acquire the directory lock. This way we should bail if another process completed the download while we were waiting for them to release the lock.