Fix source directory permissions

[netoisc]

Closes #269

[codecov]

Codecov Report

Merging #270 into main will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
+ Coverage   80.55%   80.56%   +0.01%     
==========================================
  Files          21       21              
  Lines        1270     1271       +1     
==========================================
+ Hits         1023     1024       +1     
  Misses        247      247              

Impacted Files	Coverage Δ
src/illumidesk/apis/nbgrader_service.py	83.01% <100.00%> (+0.32%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7ac7078...36925e0. Read the comment docs.