Fix environment variable loading only working with 4 characters

[djhoese]

There was a hardcoded [5:] indexing done on environment variable names that works for any package name that is 4 letters long (ex. DASK). If the package name was longer than that then donfig wouldn't find the environment variable or rather it would assign the wrong value by not cutting enough of the prefix away.

This issue wasn't caught before because all my tests used a test config name of "test" (4 characters).

[coveralls]

Pull Request Test Coverage Report for Build 42

• 29 of 29 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.251%

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Totals
Change from base Build 41:	0.02%
Covered Lines:	1082
Relevant Lines:	1148

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💛 - Coveralls

[codecov]

Codecov Report

Merging #9 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   94.23%   94.25%   +0.02%     
==========================================
  Files           4        4              
  Lines         572      574       +2     
==========================================
+ Hits          539      541       +2     
  Misses         33       33

Impacted Files	Coverage Δ
donfig/config_obj.py	90.98% <100%> (+0.03%)	⬆️
donfig/tests/test_config.py	98.96% <100%> (ø)	⬆️

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