don't let dotnetup failure count as a install script failure

[nagilson]

Exhibited CI issue in #54752.

The exitcode set by dotnetup overrides whether the ci thinks the install script succeeded or not, when it did succeed here, but the CI thought it failed. We also now validate the folder rather than assuming exit code 0 = a happy path (I sound like claude in that sentence which is sad.)

Error: Failed to extract .NET archive for version 11.0.0-preview.6.26277.111: Could not find a part of the path 'C:\Users\cloudtest\AppData\Local\Temp\ecl2kgt3q\dotnet-62f51424-63c9-4e6a-b09b-380303c69000.tar'.
Failed to install shared frameworks (6.0, 7.0, 8.0, 9.0, 10.0, 11.0.0-preview.6.26277.111, aspnetcore@11.0.0-preview.6.26277.111) to 'D:\a\_work\1\s\.dotnet' using dotnetup (exit code '1'); falling back to dotnet install script.
GET https://builds.dotnet.microsoft.com/dotnet/scripts/v1/dotnet-install.ps1
dotnet-install: Remote file https://builds.dotnet.microsoft.com/dotnet/Runtime/6.0.0/dotnet-runtime-6.0.0-win-x64.zip size is 32516454 bytes.
dotnet-install: Downloaded file https://builds.dotnet.microsoft.com/dotnet/Runtime/6.0.0/dotnet-runtime-6.0.0-win-x64.zip size is 32516454 bytes.
dotnet-install: The remote and local file sizes are equal.
dotnet-install: Extracting the archive.
dotnet-install: Adding to current process PATH: "D:\a\_work\1\s\.dotnet\". Note: This change will not be visible if PowerShell was run as a child process.
dotnet-install: Note that the script does not ensure your Windows version is supported during the installation.
dotnet-install: To check the list of supported versions, go to https://learn.microsoft.com/dotnet/core/install/windows#supported-versions
dotnet-install: Installed version is 6.0.0
dotnet-install: Installation finished
at InstallDotNetSharedFrameworksWithInstallScript, D:\a\_work\1\s\eng\restore-toolset.ps1: line 249
at InstallDotNetSharedFrameworks, D:\a\_work\1\s\eng\restore-toolset.ps1: line 226
at InitializeCustomSDKToolset, D:\a\_work\1\s\eng\restore-toolset.ps1: line 47
at <ScriptBlock>, D:\a\_work\1\s\eng\restore-toolset.ps1: line 285
at InitializeCustomToolset, D:\a\_work\1\s\eng\common\build.ps1: line 99
at Build, D:\a\_work\1\s\eng\common\build.ps1: line 105
at <ScriptBlock>, D:\a\_work\1\s\eng\common\build.ps1: line 190
at <ScriptBlock>, D:\a\_work\_temp\3095a923-09e9-43ab-8e1e-bde4534a48b5.ps1: line 4
at <ScriptBlock>, <No file>: line 1