[cli] Add path to Python executable to 'ti diagnose'

[chengluyu]

Related issue = closes #1792

[Click here for the format server]

---

This PR adds Python executable path to ti diagnose, as mention in #1792.

There is another change: I found that running ti diagnose in development mode will cause syntax errors on macOS because python still refers to Python 2.7.16, and current executable will always be the correct one with Taichi installed. So I replaced 'python' with the executable path.

[yuanming-hu]

Very cool! LGTM. Welcome to the Taichi developer community!

I'll merge this once CI passes. Thank you for contributing!

[yuanming-hu]

Btw, could you run ti format to enforce code format? Usually @taichi-gardener will help you enforce code format, but it seems that it doesn't have access to FrokYard :-)

[codecov]

Codecov Report

Merging #1801 into master will increase coverage by 2.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1801      +/-   ##
==========================================
+ Coverage   61.74%   64.02%   +2.28%     
==========================================
  Files          19       19              
  Lines        3850     3678     -172     
  Branches      691      693       +2     
==========================================
- Hits         2377     2355      -22     
+ Misses       1348     1197     -151     
- Partials      125      126       +1     

Impacted Files	Coverage Δ
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/meta.py	62.31% <0.00%> (-0.54%)	⬇️
python/taichi/lang/__init__.py	41.94% <0.00%> (-0.51%)	⬇️
python/taichi/lang/shell.py	0.00% <0.00%> (ø)
python/taichi/lang/kernel.py	71.17% <0.00%> (+0.13%)	⬆️
python/taichi/lang/util.py	31.73% <0.00%> (+0.37%)	⬆️
python/taichi/lang/transformer.py	81.61% <0.00%> (+0.68%)	⬆️
python/taichi/lang/impl.py	66.37% <0.00%> (+1.87%)	⬆️
python/taichi/lang/snode.py	68.42% <0.00%> (+4.19%)	⬆️
... and 3 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 da41c85...d5cc9f5. Read the comment docs.

[chengluyu]

Hi, I've run ti format (and found another tiny bug and sent a PR #1802). :)