Set LIBGL_ALWAYS_SOFTWARE env variable in doc build

[greschd]

Gets rid of the MESA errors [1] which appeared since ubuntu-latest switched to 24.04.

[1] errors like the following. They seemed to ultimately not cause any build failure, but pollute the output.

MESA: error: ZINK: vkCreateInstance failed (VK_ERROR_INCOMPATIBLE_DRIVER)
glx: failed to create drisw screen

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.98%. Comparing base (f7a7b17) to head (71a247a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #762   +/-   ##
=======================================
  Coverage   94.98%   94.98%           
=======================================
  Files         109      109           
  Lines        5698     5698           
  Branches      323      323           
=======================================
  Hits         5412     5412           
  Misses        285      285           
  Partials        1        1           

Flag	Coverage Δ
direct-launch	91.57% <ø> (ø)
minimum-deps	93.14% <ø> (ø)
python-3.10	94.20% <ø> (ø)
python-3.11	94.13% <ø> (ø)
python-3.12	94.33% <ø> (ø)
server-2024R2	90.76% <ø> (ø)
server-2025R1	94.13% <ø> (ø)
server-2025R2	94.13% <ø> (ø)
server-latest	94.78% <ø> (ø)

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