Add default for RH_IS_CHECK_RELEASE

[jtpio]

Caught when running the Draft Release workflow: https://github.com/jtpio/jupyter_releaser/runs/3086332621?check_suite_focus=true

[codecov-commenter]

Codecov Report

Merging #70 (52da98a) into master (5b17742) will decrease coverage by 2.04%.
The diff coverage is 68.78%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   89.03%   86.99%   -2.05%     
==========================================
  Files          15       15              
  Lines        1697     1776      +79     
  Branches      198      215      +17     
==========================================
+ Hits         1511     1545      +34     
- Misses        118      151      +33     
- Partials       68       80      +12     

Impacted Files	Coverage Δ
jupyter_releaser/actions/draft_release.py	0.00% <0.00%> (ø)
jupyter_releaser/util.py	82.19% <37.50%> (-2.71%)	⬇️
jupyter_releaser/changelog.py	88.18% <62.50%> (-2.56%)	⬇️
jupyter_releaser/lib.py	74.70% <67.56%> (-3.04%)	⬇️
jupyter_releaser/npm.py	80.14% <70.37%> (-3.60%)	⬇️
jupyter_releaser/tests/test_cli.py	97.64% <91.66%> (-0.20%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/cli.py	92.37% <100.00%> (+0.13%)	⬆️
jupyter_releaser/python.py	91.66% <100.00%> (+3.78%)	⬆️
jupyter_releaser/tests/conftest.py	98.50% <100.00%> (+0.01%)	⬆️
... and 1 more

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[blink1073]

Thanks!