Use correct TFM for source build when OfficialBuildId isn't set

[mthalman]

Related to dotnet/source-build#4855

When building fsharp in source build without having OfficialBuildId set, it ends up using the default TFM of net9.0 for the FSharp.Compiler.Service project. This causes a failure because source build uses the latest TFM, net10.0 currently:

System.Collections.Immutable 10.0.0-preview.2.25160.1 doesn't support net9.0 and has not been tested with it. Consider upgrading your TargetFramework to net10.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk.

This is caused by an incorrect property being used for setting TargetFrameworks when OfficialBuildId isn't set for the FSharp.Compiler.Service project. It's configured to use FSharpNetCoreProductDefaultTargetFramework, which is always hardcoded:

fsharp/Directory.Build.props

Line 9 in 3259edf

<FSharpNetCoreProductDefaultTargetFramework>net9.0</FSharpNetCoreProductDefaultTargetFramework>

Instead, it should use FSharpNetCoreProductTargetFramework, which will be set to FSharpNetCoreProductDefaultTargetFramework unless source build is being used when it'll be set to NetCurrent:

fsharp/Directory.Build.props

Lines 75 to 90 in 3259edf

	<Choose>
	<!-- Once we move to OOP in VS, and major dependants of FCS will move to netcore (not netstandard),
	we should also support $(NetPrevious) for all releases.
	This will likely include FCS and FSharp.Core as well as shipped products.
	Right now, it only covers products we ship (FSC and FSI), not NuGet packages. -->
	<When Condition="'$(DotNetBuildSourceOnly)' == 'true' AND '$(DotNetBuildOrchestrator)' == 'true'">
	<PropertyGroup>
	<FSharpNetCoreProductTargetFramework>$(NetCurrent)</FSharpNetCoreProductTargetFramework>
	</PropertyGroup>
	</When>
	<Otherwise>
	<PropertyGroup>
	<FSharpNetCoreProductTargetFramework>$(FSharpNetCoreProductDefaultTargetFramework)</FSharpNetCoreProductTargetFramework>
	</PropertyGroup>
	</Otherwise>
	</Choose>

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