fix(cli): Fix appset generate in --core mode

[OpenGuidou]

Fixes #20692

I'd like it to be cherry-picked to 2.13.

Checklist:

• [x] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• [x] The title of the PR states what changed and the related issues number (used for the release note).
• [x] The title of the PR conforms to the Toolchain Guide
• [x] I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• [x] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• [x] Does this PR require documentation updates?
• [x] I've updated documentation as required by this PR.
• [x] I have signed off all my commits as required by DCO
• [x] I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• [x] My build is green (troubleshooting builds).
• [x] My new feature complies with the feature status guidelines.
• [x] I have added a brief description of why this PR is necessary and/or what this PR solves.
• [x] Optional. My organization is added to USERS.md.
• [x] Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[bunnyshell[bot]]

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[speedfl]

LGTM

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 55.16%. Comparing base (993d79c) to head (8de96ed). Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/headless/headless.go	0.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #20717 +/- ## ========================================== - Coverage 55.21% 55.16% -0.05% ========================================== Files 324 324 Lines 55256 55263 +7 ========================================== - Hits 30508 30485 -23 - Misses 22133 22160 +27 - Partials 2615 2618 +3 ```

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[OpenGuidou]

/cherry-pick release-2.13