Fix building docs

[sclu1034]

One of the new build scripts doesn't use the CMake LUA_EXECUTABLE and instead relies on /usr/bin/env lua.
On systems where multiple Lua versions are installed, that can lead to inconsistencies.
On systems where 5.4 is the default, for which LGI doesn't have a release, that will lead to

lua: ...projects/awesome/tests/examples/_postprocess_cleanup.lua:8: module 'lgi' not found:
        No LuaRocks module found for lgi
        no field package.preload['lgi']
        no file '/home/lucas/.luarocks/share/lua/5.4/lgi.lua'
        no file '/home/lucas/.luarocks/share/lua/5.4/lgi/init.lua'
        no file '/usr/share/lua/5.4/lgi.lua'
        no file '/usr/share/lua/5.4/lgi/init.lua'
        no file './lgi.lua'
        no file './lgi/init.lua'
        no file '/usr/lib/lua/5.4/lgi.lua'
        no file '/usr/lib/lua/5.4/lgi/init.lua'
        no file '/home/lucas/.luarocks/lib/lua/5.4/lgi.so'
        no file '/usr/lib/lua/5.4/lgi.so'
        no file '/usr/local/lib/lua/5.4/lgi.so'
        no file './lgi.so'
        no file '/usr/lib/lua/5.4/loadall.so'
stack traceback:
        [C]: in function 'require'
        ...projects/awesome/tests/examples/_postprocess_cleanup.lua:8: in main chunk
        [C]: in ?
make[3]: *** [CMakeFiles/DOC_EXAMPLES_PPOSTPROCESS_CLEANUP.dir/build.make:70: CMakeFiles/DOC_EXAMPLES_PPOSTPROCESS_CLEANUP] Error 1
make[2]: *** [CMakeFiles/Makefile2:28711: CMakeFiles/DOC_EXAMPLES_PPOSTPROCESS_CLEANUP.dir/all] Error 2
make[1]: *** [Makefile:136: all] Error 2
make: *** [Makefile:15: cmake-build] Error 2

[codecov]

Codecov Report

Merging #3715 (d640314) into master (b16f628) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3715   +/-   ##
=======================================
  Coverage   90.91%   90.91%           
=======================================
  Files         896      896           
  Lines       56633    56633           
=======================================
+ Hits        51489    51490    +1     
+ Misses       5144     5143    -1     

Flag	Coverage Δ
gcov	90.91% <ø> (+<0.01%)	⬆️
luacov	93.67% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/awful/widget/layoutlist.lua	89.88% <0.00%> (+0.59%)	⬆️

[Elv13]

Thanks for the fix. We should probably have a way to catch those in the CI. It's easy to regress by mistake...