Update jenkinsfile

[hejm37]

The update of Jenkinsfile includes:

• Use https aliyun pip source mirror;
• Explicitly install Cython using pip to avoid CI failure;
• Remove build directory before copying from source directory.

[codecov]

Codecov Report

Merging #444 into master will decrease coverage by 1.57%.
The diff coverage is 29.41%.

@@            Coverage Diff             @@
##           master     #444      +/-   ##
==========================================
- Coverage   63.99%   62.42%   -1.58%     
==========================================
  Files         124      126       +2     
  Lines        6933     7116     +183     
  Branches     1217     1255      +38     
==========================================
+ Hits         4437     4442       +5     
- Misses       2291     2469     +178     
  Partials      205      205              

Flag	Coverage Δ
#unittests	62.42% <29.41%> (-1.58%)	⬇️

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Impacted Files	Coverage Δ
mmcv/runner/hooks/sync_buffer.py	0.00% <0.00%> (ø)
mmcv/runner/base_runner.py	66.83% <100.00%> (+0.84%)	⬆️
mmcv/onnx/onnx_utils/symbolic_helper.py	0.00% <0.00%> (ø)

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