Fix isolated-projects incompatibility from dynamic property lookup

[Sharang-1]

Problem

With Gradle's Isolated Projects enabled
(-Dorg.gradle.unsafe.isolated-projects=true), applying the SKIE plugin
fails configuration with:

Plugin 'co.touchlab.skie': Project ':app' cannot dynamically look up a
property in the parent project ':'

The configuration-cache report points at
SkieKotlinVariantResolver.findKotlinGradlePluginVersionFromOverrideProperty
(line 103), which uses Project.findProperty("skie.kgpVersion"). That API
walks up to the parent project's properties, which Isolated Projects
forbids because it couples configuration of the subproject to the root.

Fix

Replace findProperty(...) as? String with
providers.gradleProperty(...).orNull. The new API:

• Reads the same surface: gradle.properties at root and subproject, plus
  -P CLI properties — so users who set skie.kgpVersion anywhere still
  have it picked up.
• Returns a Provider<String> resolved through Gradle's lazy
  value-supplier model, which is the documented Isolated Projects-safe
  replacement for dynamic findProperty calls.

The fixed private helper is the only path to skie.kgpVersion, so this
one-line change covers both call sites that need the override (warning
emission and the actual version lookup).

Reproduction

In any KMP project with SKIE applied:

./gradlew help -Dorg.gradle.unsafe.isolated-projects=true

• Before: BUILD FAILED with the error quoted above.
• After: BUILD SUCCESSFUL, configuration cache entry stored.

Also verified end-to-end with the iOS link task; no further isolation
violations surfaced in the configuration-cache report:

./gradlew :shared:linkDebugFrameworkIosArm64 -Dorg.gradle.unsafe.isolated-projects=true