Fix build by locally building JSpecify conformance artifacts

[wmdietl]

Largely with the help of Claude from the CI failure.

[Copilot]

Pull request overview

This PR aims to fix CI/build breakages by making resolution of the JSpecify conformance snapshot more robust, ensuring required test dependencies are declared explicitly, and correcting a few build/test runner and documentation details.

Changes:

• Add mavenLocal() fallback plus repository content scoping for the org.jspecify.conformance snapshot artifacts, and declare an explicit Guava test dependency.
• Fix dependency path parsing in the conformance test runner by omitting empty entries.
• Update CI to build/install the JSpecify snapshot to the local Maven repo, and fix a README Gradle command typo.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/test/java/conformance/NullnessJSpecifyConformanceTest.java	Improves deps parsing by omitting empty path entries when splitting.
README.md	Fixes the Gradle command typo (assemble).
build.gradle	Adds mavenLocal() fallback + scoped snapshots repo, and explicitly depends on Guava for tests.
.github/workflows/workflow.yml	Builds/publishes the JSpecify snapshot into Maven Local during CI and fixes the matrix variable name in a step label.

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