Fix handling of workspace paths

[blink1073]

Follow up to #53 to fix all usages of workspace paths.

[codecov-commenter]

Codecov Report

Merging #56 (1298c00) into master (9129099) will decrease coverage by 0.29%.
The diff coverage is 79.06%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
- Coverage   88.51%   88.21%   -0.30%     
==========================================
  Files          15       15              
  Lines        1715     1748      +33     
  Branches      205      211       +6     
==========================================
+ Hits         1518     1542      +24     
- Misses        122      127       +5     
- Partials       75       79       +4     

Impacted Files	Coverage Δ
jupyter_releaser/lib.py	75.29% <66.66%> (-1.07%)	⬇️
jupyter_releaser/npm.py	80.14% <76.08%> (-0.48%)	⬇️
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%> (+0.09%)	⬆️
jupyter_releaser/util.py	85.00% <100.00%> (+0.10%)	⬆️

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