Remove Arcade net9.0 workarounds (#17987)

Author: mthalman

src/SourceBuild/content/Directory.Build.props

@@ -8,8 +8,6 @@
 
   <Import Condition="'$(SkipArcadeSdkImport)' != 'true'" Project="Sdk.props" Sdk="Microsoft.DotNet.Arcade.Sdk" />
   <PropertyGroup>
-    <!-- TODO: Remove NetCurrent override: https://github.com/dotnet/source-build/issues/3743 -->
-    <NetCurrent>net9.0</NetCurrent>
     <!-- Fake, for SDK. -->
     <TargetFramework>netstandard2.0</TargetFramework>
     <!-- We have no projects targeting multiple frameworks, so don't include in output path. -->

src/SourceBuild/content/build.sh

@@ -259,7 +259,12 @@ if [ "$alternateTarget" == "true" ]; then
   export NUGET_PACKAGES=$NUGET_PACKAGES/smoke-tests
   "$CLI_ROOT/dotnet" msbuild "$SCRIPT_ROOT/build.proj" -bl:"$SCRIPT_ROOT/artifacts/log/Debug/BuildTests_$LogDateStamp.binlog" -flp:"LogFile=$SCRIPT_ROOT/artifacts/logs/BuildTests_$LogDateStamp.log" -clp:v=m ${MSBUILD_ARGUMENTS[@]} "$@"
 else
-  "$CLI_ROOT/dotnet" msbuild "$SCRIPT_ROOT/eng/tools/init-build.proj" -bl:"$SCRIPT_ROOT/artifacts/log/Debug/BuildXPlatTasks_$LogDateStamp.binlog" -flp:LogFile="$SCRIPT_ROOT/artifacts/logs/BuildXPlatTasks_$LogDateStamp.log" -t:PrepareOfflineLocalTools ${MSBUILD_ARGUMENTS[@]} "$@"
+  # BuildXPlatTasks uses NetCurrent but that is not set since Arcade isn't used here.
+  # Bootstrap NetCurrent by deriving it from the installed .NET CLI version.
+  netCurrent="$($CLI_ROOT/dotnet --version | while IFS='.' read major minor _; do echo "net$major.$minor"; done)"
+
+  "$CLI_ROOT/dotnet" msbuild "$SCRIPT_ROOT/eng/tools/init-build.proj" -p:NetCurrent=$netCurrent -bl:"$SCRIPT_ROOT/artifacts/log/Debug/BuildXPlatTasks_$LogDateStamp.binlog" -flp:LogFile="$SCRIPT_ROOT/artifacts/logs/BuildXPlatTasks_$LogDateStamp.log" -t:PrepareOfflineLocalTools ${MSBUILD_ARGUMENTS[@]} "$@"
+
   # kill off the MSBuild server so that on future invocations we pick up our custom SDK Resolver
   "$CLI_ROOT/dotnet" build-server shutdown