[xaprepare] Improve dotnet-install script logging

[pjcollins]

Context: #8311 (comment)

Improves the logging around attempts to download or calculate the size
of the dotnet-install script. Messages about cached install script use
will be more explicit, and readability of failure cases should also be
improved.

[jonathanpeppers]

CI skips artifacts when green:

Evaluating: or(ne(variables['Agent.JobStatus'], 'Succeeded'), eq(variables['XA.PublishAllLogs'], 'true'))
Expanded: or(ne('Succeeded', 'Succeeded'), eq('false', 'true'))
Result: False

But I tested locally and this looks good 👍:

[00:00:02.2208928] =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[00:00:02.2210287] Install required .NET Preview SDK locally
[00:00:02.2211579] =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[00:00:02.2212942] 
[00:00:02.2244875] Deleting directory recursively: D:\src\xamarin-android\bin\Debug\dotnet
[00:00:02.2247367] Deleting directory D:\src\xamarin-android\bin\Debug\dotnet (recursively? True)
[00:00:02.2688202] Downloading dotnet-install script...
[00:00:12.2808715]   -> https://dot.net/v1/dotnet-install.ps1
[00:00:12.9513108] dotnet-install URL: https://dot.net/v1/dotnet-install.ps1
[00:00:12.9513157] About to start downloading
[00:00:12.9522919] Calling GetAsync
[00:00:13.2957439] GetAsync finished
[00:00:13.2968682] Downloading...
[00:00:13.3078922] success == True[00:00:24.0220820] Copying file 'D:\android-archives\dotnet-install.ps1-tmp' to 'D:\android-archives\dotnet-install.ps1' (overwrite destination: True)
[00:00:24.4545120] Deleting file: D:\android-archives\dotnet-install.ps1-tmp
[00:00:53.1241716] Creating directory: D:\src\xamarin-android\bin\Debug\dotnet
[00:00:53.1247684] Copying file 'D:\android-archives\dotnet-install.ps1' to 'D:\src\xamarin-android\bin\Debug\dotnet\dotnet-install.ps1' (overwrite destination: True)
[00:00:53.1299774] Discovering download URLs for dotnet SDK '8.0.100-rc.2.23422.31'...
[00:00:53.1417920] Running: powershell.exe "-NoProfile" "-ExecutionPolicy" "unrestricted" "-file" "D:\src\xamarin-android\bin\Debug\dotnet\dotnet-install.ps1" "-Version" "8.0.100-rc.2.23422.31" "-InstallDir" "D:\src\xamarin-android\bin\Debug\dotnet" "-Verbose" "-DryRun"
[00:00:54.8692556] Installing dotnet SDK '8.0.100-rc.2.23422.31'...
[00:00:54.8700627] Unpacking D:\android-archives\dotnet-sdk-8.0.100-rc.2.23422.31-win-x64.zip to directory: D:\src\xamarin-android\bin\Debug\dotnet
[00:00:54.8702760] Custom NuGet path: D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe
[00:00:54.8703101] 7zip executable path is rooted, using verbatim
[00:00:54.8703112] Full 7zip executable path value: D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe
[00:00:54.8709701] Attempting to find version fetcher for: D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe (D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe)
[00:00:54.8739036] Fetcher not found
[00:00:54.8739172] Attempting to find version fetcher for: 7za.exe (D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe)
[00:00:54.8739179] Fetcher not found
[00:00:54.8748188] Archive path: D:\android-archives\dotnet-sdk-8.0.100-rc.2.23422.31-win-x64.zip
[00:00:54.8759313] [7zip] extracting archive
[00:00:54.8773918] [7zip] log file: [00:00:54.8775458] bin\BuildDebug\prepare-20230830T161259.7zip-extract.dotnet-sdk-8.0.100-rc.2.23422.31-win-x64.zip.log
[00:00:54.8783086] Running: D:\.nuget\packages\7-zip.commandline\18.1.0\tools\x64\7za.exe "x" -bd -y "-oD:\src\xamarin-android\bin\Debug\dotnet" "D:\android-archives\dotnet-sdk-8.0.100-rc.2.23422.31-win-x64.zip"
[00:01:03.1541902] Elapsed: 00:00:08.2755312[00:01:03.1544857]