Update ci pipeline

[mauvais2]

• Changed to pin a specific ubuntu version rather than ubuntu:latest to avoid buggy release(s)
• Updated requirements, to use --find-links instead of --extra-index-url to avoid Dependabot think it’s a PyPI-style index and cause the authentication failure (the link doesn't require authentication when access) to avoid the dependabot issue like this. Will need to test, verify in master branch.
  ⁠

Dependabot encountered '1' error(s) during execution, please check the logs for more details.
+----------------------------------------------------------------------------------------------------------+
|                                      Dependencies failed to update                                       |
+------------+---------------------------------------+-----------------------------------------------------+
| Dependency | Error Type                            | Error Details                                       |
+------------+---------------------------------------+-----------------------------------------------------+
| torch      | private_source_authentication_failure | {                                                   |
|            |                                       |   "source": "https://data.dgl.ai/wheels/repo.html/" |
|            |                                       | }                                                   |
+------------+---------------------------------------+-----------------------------------------------------+

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
+ Coverage   23.49%   27.04%   +3.55%     
==========================================
  Files          51       51              
  Lines       13630    13630              
==========================================
+ Hits         3202     3686     +484     
+ Misses      10428     9944     -484     

see 11 files with indirect coverage changes

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