Allow easier overriding of crossgen tasks/targets

[elinor-fung]

• Remove ResolveReadyToRunCompilers from Microsoft.NET.Publish.targets. This was always overriding (and identical to) the one in Microsoft.NET.CrossGen.targets
• Add properties for CrossGenTargetsPath and CrossGenTasksPath that can be set to import a different targets file and use a different task assembly. This is more in line with what we do for ILLink and should allow simplification of how dotnet/runtime uses in-repo crossgen targets in dotnet/runtime

[Copilot]

Pull request overview

This PR improves the extensibility of ReadyToRun crossgen tasks and targets by introducing customizable path properties. It removes duplicate target definitions and establishes a consistent pattern for overriding build behavior.

Key Changes

• Introduced CrossGenTargetsPath property to allow customization of the CrossGen targets file import location
• Introduced CrossGenTasksPath property to allow customization of the task assembly used for ReadyToRun compilation
• Removed duplicate ResolveReadyToRunCompilers target from Microsoft.NET.Publish.targets that was overriding the identical definition in Microsoft.NET.CrossGen.targets

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets	Added CrossGenTargetsPath property and updated import statement to use it instead of hardcoded path
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Publish.targets	Removed duplicate ResolveReadyToRunCompilers target definition (now only defined in CrossGen.targets)
src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.CrossGen.targets	Added CrossGenTasksPath property and updated all three UsingTask declarations to reference it