Change mv to cp when downloading softlinks.

[AnguseZhang]

If the file to be downloaded is a softlink, cp should be performed instead of mv. Otherwise, lfile is still a file linked to some original file, and when this file's removed, lfile will be invalid.

[codecov-commenter]

Codecov Report

Merging #562 (c8407e8) into devel (11ce228) will increase coverage by 0.12%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##            devel     #562      +/-   ##
==========================================
+ Coverage   32.91%   33.03%   +0.12%     
==========================================
  Files          86       86              
  Lines       14629    14739     +110     
==========================================
+ Hits         4815     4869      +54     
- Misses       9814     9870      +56     

Impacted Files	Coverage Δ
dpgen/dispatcher/LocalContext.py	89.47% <66.66%> (-0.53%)	⬇️
dpgen/generator/run.py	65.80% <0.00%> (-0.69%)	⬇️
dpgen/data/gen.py	51.04% <0.00%> (-0.17%)	⬇️
dpgen/dispatcher/ALI.py	0.00% <0.00%> (ø)
dpgen/remote/decide_machine.py	100.00% <0.00%> (ø)
dpgen/generator/lib/gaussian.py	31.57% <0.00%> (+0.19%)	⬆️
dpgen/generator/lib/lammps.py	72.59% <0.00%> (+4.32%)	⬆️

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[AnguseZhang]

@felix5572 This should also be applied to dpdispatcher.