make CI more debug-friendly

[szha]

Description

• force install in CI
• print xargs commands
• fix pandoc version, since the utils.pandoc in nbconvert (dep of nbsphinx) has max version of 2.0.0 https://github.com/jupyter/nbconvert/blob/master/nbconvert/utils/pandoc.py#L18

[codecov]

Codecov Report

Merging #312 into master will increase coverage by 2.5%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #312     +/-   ##
=========================================
+ Coverage   77.29%   79.79%   +2.5%     
=========================================
  Files          80       80             
  Lines        7077     6503    -574     
  Branches     1168     1022    -146     
=========================================
- Hits         5470     5189    -281     
+ Misses       1346     1062    -284     
+ Partials      261      252      -9

Impacted Files	Coverage Δ
scripts/nmt/utils.py	78.57% <0%> (-15.55%)	⬇️
scripts/nmt/loss.py	61.01% <0%> (-10.75%)	⬇️
gluonnlp/model/__init__.py	89.47% <0%> (-10.53%)	⬇️
scripts/nmt/bleu.py	84.09% <0%> (-4.68%)	⬇️
conftest.py	79.24% <0%> (-3.78%)	⬇️
scripts/sentiment_analysis/sentiment_analysis.py	86.36% <0%> (-2.53%)	⬇️
gluonnlp/vocab/vocab.py	94.48% <0%> (-0.69%)	⬇️
scripts/language_model/word_language_model.py	0% <0%> (-0.31%)	⬇️
scripts/nmt/_constants.py	100% <0%> (ø)	⬆️
scripts/language_model/cache_language_model.py	0% <0%> (ø)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 70bb562...6d31d55. Read the comment docs.

[mli]

Job PR-312/5 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-312/5/index.html

[mli]

Job PR-312/6 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-312/6/index.html