FIX: Remove GH Action set-env

[mgxd]

https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7a98dd7). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #64   +/-   ##
=========================================
  Coverage          ?   88.07%           
=========================================
  Files             ?        5           
  Lines             ?      218           
  Branches          ?        0           
=========================================
  Hits              ?      192           
  Misses            ?       26           
  Partials          ?        0           

Flag	Coverage Δ
api	88.07% <ø> (?)
config	54.58% <ø> (?)
datalad	78.44% <ø> (?)
dls3	56.42% <ø> (?)
s3	59.63% <ø> (?)

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Impacted Files	Coverage Δ
templateflow/conf/bids.py	100.00% <0.00%> (ø)
templateflow/__init__.py	100.00% <0.00%> (ø)
templateflow/conf/__init__.py	89.09% <0.00%> (ø)
templateflow/api.py	78.65% <0.00%> (ø)
templateflow/conf/_s3.py	97.91% <0.00%> (ø)

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