chore: Rename environment variables to avoid spurious Scrapy deprecation warnings

[jpmckinney]

Partially addresses #369

[codecov[bot]]

Codecov Report

Merging #438 (0f96df7) into master (83aec7a) will not change coverage. The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #438   +/-   ##
=======================================
  Coverage   79.67%   79.67%           
=======================================
  Files          23       23           
  Lines        1053     1053           
=======================================
  Hits          839      839           
  Misses        214      214           

Flag	Coverage Δ
unittests	79.67% <66.66%> (ø)

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Impacted Files	Coverage Δ
scrapyd/utils.py	88.23% <0.00%> (ø)
scrapyd/environ.py	82.22% <75.00%> (ø)
scrapyd/runner.py	97.22% <100.00%> (ø)

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[jpmckinney]

Superseded by #471