Use unique name for bootstrap artifacts

[tmat]

To avoid errors like https://dev.azure.com/dnceng-public/public/_build/results?buildId=352872&view=logs&j=7cdae500-0ea5-58ba-9407-8d2db153220d&t=aab43132-2cb6-5799-5ba5-bd2e21456d46:

Chunk '98' attempt '1' of file 'Bootstrap Packages - PreRelease/Microsoft.CodeAnalysis.LanguageServer.4.8.0-ci.final.nupkg' fail to send request to server. Error: Microsoft.VisualStudio.Services.WebApi.VssServiceResponseException: Blob is incomplete (missing block). Blob: 437758d3ec2bee11a9bb000d3a949e90, Expected Offset: 0, Actual Offset: 4194304
---> System.InvalidOperationException: Blob is incomplete (missing block). Blob: 437758d3ec2bee11a9bb000d3a949e90, Expected Offset: 0, Actual Offset: 4194304
   --- End of inner exception stack trace ---
   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.HandleResponseAsync(HttpResponseMessage response, CancellationToken cancellationToken)
   at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.SendAsync(HttpRequestMessage message, HttpCompletionOption completionOption, Object userState, CancellationToken cancellationToken)
   at Microsoft.VisualStudio.Services.FileContainer.Client.FileContainerHttpClient.UploadFileAsync(Int64 containerId, String itemPath, Stream fileStream, Byte[] contentId, Int64 fileLength, Boolean isGzipped, Guid scopeIdentifier, CancellationToken cancellationToken, Int32 chunkSize, Int32 chunkRetryTimes, Boolean uploadFirstChunk, Object userState).