add another way to load LAMMPS plugins

[njzjz]

by setting the environmental variable LAMMPS_PLUGIN_PATH

[codecov-commenter]

Codecov Report

Merging #1604 (95b0d0e) into devel (785cfa7) will decrease coverage by 12.27%.
The diff coverage is n/a.

❗ Current head 95b0d0e differs from pull request most recent head 4709667. Consider uploading reports for the commit 4709667 to get more accurate results

@@             Coverage Diff             @@
##            devel    #1604       +/-   ##
===========================================
- Coverage   76.56%   64.28%   -12.28%     
===========================================
  Files          93        5       -88     
  Lines        7817       14     -7803     
===========================================
- Hits         5985        9     -5976     
+ Misses       1832        5     -1827     

Impacted Files	Coverage Δ
deepmd/infer/deep_wfc.py
source/op/_prod_virial_se_r_grad.py
deepmd/descriptor/hybrid.py
deepmd/infer/deep_polar.py
deepmd/utils/tabulate.py
deepmd/descriptor/se_a_ef.py
deepmd/utils/data_system.py
deepmd/__about__.py
deepmd/utils/network.py
deepmd/utils/weight_avg.py
... and 78 more

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