[fix](build) Keep relocated storage sources out of unity builds

[hoshinojyunn]

What problem does this PR solve?

Issue Number: None

Related PR: #66052

Problem Summary:

PR #66052 moved collection_statistics.cpp from be/src/storage/compaction/ to be/src/storage/index/inverted/similarity/. The Storage unity-build skip list still used the old path. The doris_skip_unity_inclusion helper intentionally rejects nonexistent entries, so a clean BE CMake configuration fails before compilation.

The relocated source must remain excluded from the unity batch. After updating the path, CMake repacks the remaining Storage sources and puts variant_assembler.cpp and variant_storage_cell.cpp in one generated unity translation unit. Both files define an implementation-local publish_encoded helper in an anonymous namespace; this is valid when compiled independently but becomes a redefinition in one unity translation unit.

This PR updates the collection-statistics exclusion to its new source path and excludes variant_storage_cell.cpp from unity builds, preserving the independent compilation boundary for both Variant files. It changes only build grouping and does not alter runtime behavior.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
    • Run DORIS_HOME=/mnt/disk1/ganderun/doris cmake --build be/build_Release --target Storage -j192.
    • Verify CMake accepts the relocated skip entry and the Storage target builds with unity builds enabled.
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[hoshinojyunn]

run buildall