[Enhance] Skip CI when some specific files were changed

[ckkelvinchan]

Motivation

Skip CI when some specific files were changed to save the CI resources.

Reference: open-mmlab/mmcv#1533

Modifications

1. Update .github/workflows/build.yml
2. Add .github/workflows/lint.yml

[codecov]

Codecov Report

Merging #640 (ec164c4) into master (ba9d8e5) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
- Coverage   80.37%   80.21%   -0.16%     
==========================================
  Files         197      197              
  Lines       11126    11038      -88     
  Branches     1771     1771              
==========================================
- Hits         8942     8854      -88     
  Misses       1912     1912              
  Partials      272      272              

Flag	Coverage Δ
unittests	80.19% <ø> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ackbones/encoder_decoders/pconv_encoder_decoder.py	43.47% <0.00%> (-2.36%)	⬇️
mmedit/models/base.py	82.75% <0.00%> (-2.09%)	⬇️
...ones/encoder_decoders/two_stage_encoder_decoder.py	53.84% <0.00%> (-1.16%)	⬇️
...dels/backbones/encoder_decoders/encoders/resnet.py	66.01% <0.00%> (-0.97%)	⬇️
...s/backbones/encoder_decoders/gl_encoder_decoder.py	81.48% <0.00%> (-0.67%)	⬇️
mmedit/datasets/pipelines/normalization.py	88.88% <0.00%> (-0.59%)	⬇️
mmedit/datasets/pipelines/augmentation.py	85.65% <0.00%> (-0.51%)	⬇️
mmedit/models/mattors/indexnet.py	84.37% <0.00%> (-0.48%)	⬇️
mmedit/models/mattors/base_mattor.py	84.94% <0.00%> (-0.48%)	⬇️
mmedit/models/mattors/dim.py	82.35% <0.00%> (-0.34%)	⬇️
... and 21 more

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