Add --no-sync option to update command

[frostming]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Close #684

[codecov-commenter]

Codecov Report

Merging #688 (29394d1) into main (4df049f) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #688      +/-   ##
==========================================
- Coverage   83.66%   83.64%   -0.02%     
==========================================
  Files          73       73              
  Lines        6114     6116       +2     
  Branches     1083     1084       +1     
==========================================
+ Hits         5115     5116       +1     
  Misses        707      707              
- Partials      292      293       +1     

Flag	Coverage Δ
unittests	83.43% <80.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
pdm/cli/actions.py	75.69% <50.00%> (-0.24%)	⬇️
pdm/cli/commands/update.py	100.00% <100.00%> (ø)
pdm/models/requirements.py	92.60% <100.00%> (ø)

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