Fix Gradle configuration cache incompatibility in conformanceTests task

[Copilot]

conformanceTests fails when Gradle's configuration cache is enabled (org.gradle.configuration-cache=true) because fileTree() — a delegate to the Project object — is called inside a doFirst block (execution time), which Gradle cannot serialize.

Task :conformanceTests FAILED
Invocation of 'fileTree' references a Gradle script object from a Groovy closure
at execution time, which is unsupported with the configuration cache.

Change

Replace fileTree("${conformanceTests}/deps").join(":") in the doFirst block with a plain File.eachFileRecurse traversal — no Gradle project-object reference, same result (all files collected recursively, sorted alphabetically to match fileTree's default ordering):

// Before
doFirst {
    def deps = fileTree("${conformanceTests}/deps").join(":")
    systemProperties(["JSpecifyConformanceTest.deps": deps, ...])
}

// After
doFirst {
    def depsDir = new File("${conformanceTests}/deps")
    def depsList = []
    if (depsDir.exists()) {
        depsDir.eachFileRecurse { f -> if (f.isFile()) depsList << f.absolutePath }
        depsList.sort()
    }
    systemProperties(["JSpecifyConformanceTest.deps": depsList.join(":"), ...])
}

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[wmdietl]

Build and demo tests work without issue again.