Build cache entry from openapi-generator-mill-plugin:surefire:test is not relocatable

Context

• The openapi-generator project is integrated with Develocity
• Develocity provides build acceleration through its build cache feature
• The cache key is based on the goal inputs
• Surefire's test goal defines environmentVariables and testClasspathElements as goal inputs (among others)

Issue

The build cache usage is not optimal as the openapi-generator-mill-plugin:surefire:test goal cache entry can't be relocated.
A cache entry is considered relocatable if its output can be reused when the task is executed in a different location.

This is surfaced in a Develocity build scan comparison of 2 builds run from different locations:

The goal's inputs have references to absolute paths:

• environmentVariables: MILL_TEST_RESOURCE_DIR is set to a subfolder of ${project.basedir} here
• testClasspathElements: openapi-generator/modules/openapi-generator-mill-plugin/target/classes/org/openapitools/generator/mill contains some generated classes with absolute paths in their definition

public interface OpenApiModule extends Module {
    [...]

    default Task.Command<BoxedUnit> validateOpenapiSpec(final String spec, final boolean recommend, final boolean failOnWarnings) {
        return new Task.Command(.MODULE$.Nil(), (items, ctx) -> {
            [...]
        }, [...], sourcecode.File..MODULE$.apply("<THIS_IS_AN_ABSOLUTE_PATH>/modules/openapi-generator-mill-plugin/src/main/scala/org/openapitools/generator/mill/OpenApiModule.scala"), [...]

Impact

The cache entries are populated in the Develocity cache node from CI, therefore remote cache hit can't happen for local developers.
In my experience, the goal duration takes around 5 seconds. When possible, this wasted time could be almost eliminated.

Start in modules/openapi-generator-mill-plugin/pom.xml around line 171 and inspect the surefire:test inputs, especially MILL_TEST_RESOURCE_DIR. Then inspect the generated classes under target/classes/org/openapitools/generator/mill and the Scala source path shown in the issue; done means the goal's cache entry is relocatable across build locations.