flaky test: TestPromptExitCode/plugin_upgrade, estPromptExitCode/install

Description

This test was recently rewritten from testing plugin upgrade with
DCT enabled to just "plugin upgrade", but there's a fair amount of
complexity in the e2e tests that set up different daemons and registries.

It's possible that tests are affecting each-other, and some state (config)
is left behind.

Let's skip the test for now, and add a tracking ticket to dig deeper.

=== FAIL: e2e/global TestPromptExitCode/plugin_upgrade (7.55s)
    cli_test.go:205: assertion failed: 
        Command:  docker plugin push registry:5000/plugin-upgrade-test:latest
        ExitCode: 1
        Error:    exit status 1
        Stdout:   The push refers to repository [registry:5000/plugin-upgrade-test]
        459089aa5943: Preparing
        adc41078d1d9: Preparing
        d7bff979db13: Preparing
        459089aa5943: Preparing
        
        Stderr:   error pushing plugin: failed to do request: Head "https://registry:5000/v2/plugin-upgrade-test/blobs/sha256:adc41078d1d937495df2f90444e5414a01db31e5a080f8aa4f163c64d41abd11": http: server gave HTTP response to HTTPS client
        
        
        Failures:
        ExitCode was 1 expected 0
        Expected no error

Start in cli_test.go around line 205 and locate TestPromptExitCode/plugin_upgrade, then review the related estPromptExitCode/install test and the surrounding e2e setup. Confirm the failure is caused by shared daemon, registry, or configuration state; done for this issue means the flaky test is skipped and a tracking ticket records the investigation needed.