Remove need for passed in "ArcadeSdk" variable in source build (#15834)

Instead of using a passed in variable to determine where arcade lives, use a similar strategy of a few other repos (deployment tools, runtime, diagnostics) of using the parent directory of the tasks assembly, which is a property always available. This can then be used to find the release packages targets.
Avoids the need to pass in the arcade SDK location as an env variable for FSharp.

Author: mmitche

src/Microsoft.FSharp.Compiler/Microsoft.FSharp.Compiler.fsproj

@@ -19,8 +19,8 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <ArcadeSdkDir Condition="'$(ArcadeSdkDir)' == ''">$(NuGetPackageRoot)microsoft.dotnet.arcade.sdk\$(ArcadeSdkVersion)\</ArcadeSdkDir>
-    <_BuildReleasePackagesTargets>$(ArcadeSdkDir)tools\BuildReleasePackages.targets</_BuildReleasePackagesTargets>
+    <_ArcadeSdkMSBuildProjectDir>$([System.IO.Path]::GetDirectoryName('$(ArcadeSdkBuildTasksAssembly)'))\..\</_ArcadeSdkMSBuildProjectDir>
+    <_BuildReleasePackagesTargets>$(_ArcadeSdkMSBuildProjectDir)BuildReleasePackages.targets</_BuildReleasePackagesTargets>
   </PropertyGroup>
 
   <Import Project="$(_BuildReleasePackagesTargets)" />