ci: add coverage token

[joshka]

Fixes the problem where the code coverage github action cannot upload the coverage results.

[codecov]

Codecov Report

Merging #352 (52886ed) into main (b9290b3) will decrease coverage by 0.33%.
Report is 8 commits behind head on main.
The diff coverage is 57.84%.

@@            Coverage Diff             @@
##             main     #352      +/-   ##
==========================================
- Coverage   85.08%   84.76%   -0.33%     
==========================================
  Files          40       40              
  Lines        8608     8603       -5     
==========================================
- Hits         7324     7292      -32     
- Misses       1284     1311      +27     

Files Changed	Coverage Δ
src/backend/crossterm.rs	0.00% <0.00%> (ø)
src/backend/termion.rs	27.46% <0.00%> (-0.59%)	⬇️
src/terminal.rs	55.73% <0.00%> (-0.47%)	⬇️
src/text/masked.rs	98.64% <0.00%> (ø)
src/widgets/calendar.rs	81.06% <0.00%> (-1.25%)	⬇️
src/widgets/canvas/circle.rs	97.14% <0.00%> (ø)
src/widgets/canvas/line.rs	59.67% <0.00%> (ø)
src/widgets/canvas/map.rs	0.00% <0.00%> (ø)
src/widgets/canvas/mod.rs	91.61% <0.00%> (-0.27%)	⬇️
src/widgets/canvas/points.rs	85.71% <0.00%> (+39.56%)	⬆️
... and 25 more