Don't use deprecated buildDir (#1278)

Gradle's `$buildDir` is deprecated. This replaces it by a more modern
equivalent.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Updated build configuration to use modern Gradle APIs for resolving
build output directories, improving build compatibility, reliability,
and maintainability. This is an internal infrastructure change with no
user-facing impact.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: mernst

nullaway/build.gradle

@@ -157,7 +157,7 @@ tasks.register('buildWithNullAway', JavaCompile) {
     // Configure compilation to run with Error Prone and NullAway
     source = sourceSets.main.java
     classpath = sourceSets.main.compileClasspath
-    destinationDirectory = file("$buildDir/ignoredClasses")
+    destinationDirectory.set(layout.buildDirectory.dir('ignoredClasses'))
     options.annotationProcessorPath = files(
             configurations.errorproneExtended.asCollection(),
             sourceSets.main.annotationProcessorPath,