improve environment variable implements

[JacksonTian]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 89.92%. Comparing base (4368696) to head (13c630a).

Files	Patch %	Lines
...re/src/main/java/com/aliyuncs/utils/AuthUtils.java	60.00%	5 Missing and 1 partial :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #900      +/-   ##
============================================
- Coverage     89.98%   89.92%   -0.07%     
+ Complexity     1654     1638      -16     
============================================
  Files           124      124              
  Lines          4734     4705      -29     
  Branches        544      543       -1     
============================================
- Hits           4260     4231      -29     
- Misses          347      348       +1     
+ Partials        127      126       -1     

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