runners.feature_flags FF_USE_FASTZIP = true does not really enable fastzip

This is a snippet from my runner config

        [[runners]]
            # usual configuration
            executor = "kubernetes"
            output_limit = 32768
            builds_dir = "/builds"
            #builds_dir_is_shared = false
            environment = [
                "GIT_CLONE_PATH=$CI_BUILDS_DIR/$CI_PROJECT_PATH",
                "TRANSFER_METER_FREQUENCY=30s",
                "ARTIFACT_COMPRESSION_LEVEL=fastest", # requires FF_USE_FASTZIP
                "CACHE_COMPRESSION_LEVEL=fastest", # requires FF_USE_FASTZIP
            ]
            [runners.custom_build_dir]
                enabled = true
            [runners.feature_flags]
                FF_USE_FASTZIP = true
                FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY = true

The FF_USE_FASTZIP feature flag is enabled.

The gitlab job log shows:

Running with gitlab-runner  (7f385843)
  on gitlabsr-testing-... teftw4Ff
  feature flags: FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY:true, FF_USE_FASTZIP:true

However, I've confirmed that the helper artifacts upload is NOT using fastzip, as I see 3x faster upload speeds when manually executing the helper command.

FF_USE_FASTZIP=true  CI_SERVER_TLS_CA_FILE=/builds/jasmets/test4.tmp/CI_SERVER_TLS_CA_FILE  TRANSFER_METER_FREQUENCY=5s ARTIFACT_COMPRESSION_LEVEL=fastest   /usr/bin/gitlab-runner-helper artifacts-uploader --url https://testing.gitlabX/ --token XYZ --id 644778 --path testdir/ --expire-in 10 minutes --artifact-format zip --artifact-type archive
testdir/: found 2 matching files and directories
Uploading artifacts 630.36 MB (31.5 MB/s)

runner-helper seems to rely on the environment variable, and that appears to be set to false in the k8s pod environment.

    env:
 <snip>
    - name: FF_USE_FASTZIP
      value: "false"

To my best understand I would expect [runners.feature_flags] FF_USE_FASTZIP = true to -always- set/override the FF_USE_FASTZIP environment variable.

#28393