[wasm] GenerateWasmBootJson - close file handles

[radical]

Close files opened with File.OpenRead by utilizing the using pattern.

Background

• this was found when looking at the test failures mentioned in Update SDK aspnetcore#47540 (comment) .

The hypothesis is that the following operations happen in order:

1. GenerateBlazorBootExtensionJson calls File.Create to create obj/Release/net8.0/blazor.publish.boot-extension.json, and the file handle is closed
2. GenerateWasmBootJson opens the same file with File.OpenRead but does not close it
3. GenerateBlazorBootExtensionJson runs again as part of the nested build, but fails because (2) left the file open

The "GenerateBlazorBootExtensionJson" task failed unexpectedly. [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
System.IO.IOException: The process cannot access the file '/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/obj/Release/net8.0/blazor.publish.boot-extension.json' because it is being used by another process. [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Init(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Int64& fileLength, UnixFileMode& filePermissions) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, UnixFileMode openPermissions, Int64& fileLength, UnixFileMode& filePermissions, Func`4 createOpenException) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at System.IO.File.Create(String path) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.NET.Sdk.BlazorWebAssembly.GenerateBlazorBootExtensionJson.Execute() [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [/datadisks/disk1/work/B95509FE/w/998E08AE/e/Templates/BaseFolder/AspNet.hhjg2t3oatk0/Client/AspNet.hhjg2t3oatk0.Client.csproj]

[lewing]

/backport to release/8.0-preview4

[github-actions]

Started backporting to release/8.0-preview4: https://github.com/dotnet/runtime/actions/runs/4823593952

[lewing]

Only the osx lanes are still running and they are not impacted by this change