Setting JAR manifest encoding doesn't work with configuration cache

[mlopatkin]

Current Behavior

The specified manifest encoding on the Jar task is ignored when configuration cache is enabled

Expected Behavior

The same specified encoding is used regardless of CC state

Context (optional)

No response

Self-contained Reproducer Project

See this test:

task jar(type: Jar) {
    from file('test')
    destinationDirectory = file('dest')
    archiveFileName = 'test.jar'
    manifestContentCharset = 'ISO-8859-15'
    manifest {
        attributes 'moji': 'bak€'
    }
}

Gradle version

8.12, though it likely never worked properly

Build scan URL (optional)

No response

Your Environment (optional)

No response

[mlopatkin]

Jar task configures the manifest in a very contrived way, and the configured encoding is propagated as a side effect of saving when CC is in play:

1. There is a Cached instance in the copy spec file collection
2. It sets the encoding when computing the manifest

However, this set goes nowhere, because the same Manifest is referenced by the manifest field of the task. By the time we get to serializing the file collection, the field has been processed and the manifest has been serialized - with default encoding. Thus upon restoring from cache, the modification is lost.