hotfix: extra string at the beginning of yaml in snapshot

[elliotzh]

Description

Fix a bug that !!omap tag will generated at the beginning of flow.flex.yaml in snapshot and break local to cloud

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• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

promptflow SDK CLI Azure E2E Test Result dev/hotfix-flex-flow-yaml-issue

  4 files    4 suites   5m 0s ⏱️
238 tests 203 ✅  35 💤 0 ❌
952 runs  812 ✅ 140 💤 0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.

[github-actions]

promptflow-core test result

0 tests   0 ✅  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.

[github-actions]

SDK CLI Global Config Test Result dev/hotfix-flex-flow-yaml-issue

5 tests   5 ✅  1m 9s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.

[github-actions]

Executor Unit Test Result dev/hotfix-flex-flow-yaml-issue

792 tests   792 ✅  3m 41s ⏱️
  1 suites    0 💤
  1 files      0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.

[github-actions]

Executor E2E Test Result dev/hotfix-flex-flow-yaml-issue

241 tests   236 ✅  5m 12s ⏱️
  1 suites    5 💤
  1 files      0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.

[github-actions]

SDK CLI Test Result dev/hotfix-flex-flow-yaml-issue

    4 files      4 suites   57m 36s ⏱️
  663 tests   650 ✅ 13 💤 0 ❌
2 652 runs  2 600 ✅ 52 💤 0 ❌

Results for commit f9d9f3d.

♻️ This comment has been updated with latest results.