Fix "Install GR dependencies" step in Ubuntu CI

[alfredclwong]

Add apt-get update line to GR dependencies step

fixes #270

[codecov-commenter]

Codecov Report

Merging #271 (3352058) into main (f3bfebb) will increase coverage by 4.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   49.48%   53.93%   +4.45%     
==========================================
  Files          78       78              
  Lines        7052     7056       +4     
==========================================
+ Hits         3490     3806     +316     
+ Misses       3562     3250     -312     

Impacted Files	Coverage Δ
src/GlassCat/sources.jl	90.90% <0.00%> (+0.13%)	⬆️
src/Geometry/Transform.jl	52.73% <0.00%> (+0.32%)	⬆️
src/Geometry/Primitives/Curves/Bezier.jl	54.87% <0.00%> (+0.60%)	⬆️
src/Geometry/Primitives/NonCSG/Triangle.jl	74.64% <0.00%> (+1.40%)	⬆️
src/Geometry/CSG/Interval.jl	83.20% <0.00%> (+1.86%)	⬆️
src/Geometry/AccelSurface.jl	96.59% <0.00%> (+2.04%)	⬆️
src/Geometry/Surface.jl	68.18% <0.00%> (+4.54%)	⬆️
src/Geometry/Primitives/Cylinder.jl	87.03% <0.00%> (+7.40%)	⬆️
src/Geometry/Primitives/Sphere.jl	92.45% <0.00%> (+7.54%)	⬆️
src/Vis/Visualization.jl	8.87% <0.00%> (+8.87%)	⬆️
... and 6 more

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