fix(abc:st): fix always fixed column in group header

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1204 (631681e) into master (b480649) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1204   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files         268      268           
  Lines        8070     8072    +2     
  Branches     1478     1478           
=======================================
+ Hits         7774     7776    +2     
  Misses        135      135           
  Partials      161      161           

Impacted Files	Coverage Δ
packages/abc/st/st-column-source.ts	99.19% <100.00%> (+<0.01%)	⬆️

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