Avoid Gradle configuration cache serializer limitations by adjusting lambda captures

[embeddedt]

Summary

This PR fixes issues with RFG jar tasks when the configuration cache is enabled.

The RFG object graph has a cycle: MinecraftExtension.dependenciesForAccessTransformerScan (= compileClasspath) → resolved artifacts → DependencyDeobfuscationTransform$Parameters.fieldsCsv → provider chain from SharedMCPTasks → lambdas that capture mcExt → back to MinecraftExtension.

Gradle's configuration cache can normally serialize cycles, but appears to fail when the re-entry point is a Java-serialized lambda.

The fix is to hoist more properties into locals so the provider lambdas capture the Provider objects instead of mcExt/this. This is the same trick already used elsewhere in the code.

• mcpExtractRoot — capture mcVersion/mcpMappingChannel/mcpMappingVersion providers, not mcExt
• forgeSrgLocation — capture useForgeEmbeddedMappings and the two extract-root providers as locals
• the generateForgeSrgMappings block — hoist userdevFile(...)/mcpFile(...) calls out of the flatMap lambdas (calling them inside captures this → mcExt)

Checklist

• I have tested this PR in DevEnv
• I have tested this PR in Fullpack
• This PR is in compliance with the GTNH AI Policy
• This PR requires another PR in order to merge