Fix sky material destroy bug

[zhuxudong]

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Fix #2055

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (7a7bc45) 67.01% compared to head (5c811f1) 67.02%.

❗ Current head 5c811f1 differs from pull request most recent head 6a9408b. Consider uploading reports for the commit 6a9408b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1864   +/-   ##
=======================================
  Coverage   67.01%   67.02%           
=======================================
  Files         471      471           
  Lines       23742    23753   +11     
  Branches     3381     3384    +3     
=======================================
+ Hits        15911    15920    +9     
- Misses       6633     6635    +2     
  Partials     1198     1198           

Files	Coverage Δ
packages/core/src/mesh/MeshRenderer.ts	80.68% <0.00%> (-2.85%)	⬇️
packages/core/src/shader/ShaderProgram.ts	61.72% <0.00%> (-0.52%)	⬇️
packages/core/src/sky/Sky.ts	20.37% <0.00%> (-2.55%)	⬇️

... and 2 files with indirect coverage changes

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[cptbtptpbcptdtptp]

@zhuxudong 我之前也遇到了类似的问题
#1852
不过这块 @GuoLei1990 说应该整体设计一下，走一套方案。
#1858