Fix for java.io.File not being 'serializable' due to internal gradle cast

[alex-lmco]

Fixes #57

Only 1 test fails when I run locally but it does anyways when I run it locally without my changes:

$ ./gradlew test

> Task :test
HelmCommandsPluginTest > should not create any tasks automatically FAILED
    org.gradle.api.internal.plugins.PluginApplicationException at HelmCommandsPluginTest.kt:59
        Caused by: org.gradle.api.internal.plugins.PluginApplicationException at HelmCommandsPluginTest.kt:59
            Caused by: org.gradle.api.reflect.ObjectInstantiationException at HelmCommandsPluginTest.kt:59
                Caused by: org.gradle.api.internal.tasks.DefaultTaskContainer$TaskCreationException at HelmCommandsPluginTest.kt:59
                    Caused by: org.opentest4j.AssertionFailedError at HelmCommandsPluginTest.kt:53

330 tests completed, 1 failed

[tkrullmann]

Thanks for the work! But the issue seems to be that @InputFile must be used on RegularFileProperty task properties, instead of @Input.

Changing the annotation won't create any breaking changes, and RegularFileProperty is designed for working with file paths, so I'd rather keep it instead of changing it to a string.