Change to install numba using pip instead of conda

[kamo-naoyuki]

No description provided.

[codecov]

Codecov Report

Merging #4637 (ab20f6f) into master (36e824b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4637      +/-   ##
==========================================
- Coverage   83.09%   83.09%   -0.01%     
==========================================
  Files         517      517              
  Lines       44715    44715              
==========================================
- Hits        37157    37156       -1     
- Misses       7558     7559       +1     

Flag	Coverage Δ
test_integration_espnet1	66.36% <ø> (ø)
test_integration_espnet2	49.39% <ø> (ø)
test_python	70.91% <ø> (-0.01%)	⬇️
test_utils	23.28% <ø> (ø)

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Impacted Files	Coverage Δ
espnet/distributed/pytorch_backend/launch.py	82.75% <0.00%> (-1.15%)	⬇️

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