ci: Simplify build_llvm.bash script #4103

Job	Run time
Ubuntu latest releases gcc11 C++17 avx2 exr3.1 ocio2.2	9m 16s
Ubuntu bleeding edge gcc12 C++20 py3.10 OCIO/libtiff/exr-master boost1.74 avx2	8m 48s
Ubuntu clang14 C++20 avx2 exr3.1 ocio2.1	14m 9s
Ubuntu debug gcc7/C++14, sse4.2, exr2.4	16m 28s
Ubuntu gcc8 C++17 avx exr2.5	9m 9s
Ubuntu static libs gcc7 C++14 exr2.4	8m 57s
windows-2019 VS2019	51m 54s
macos-11 appleclang13/C++17 py3.9 MacOS-11 boost1.76 exr3.1 ocio2.1	25m 48s
macos-12 appleclang13/C++20 py3.10 MacOS-12 boost1.76 exr3.1 ocio2.1	32m 13s
macos-13 appleclang14/C++20 py3.11 MacOS-13 boost1.76 exr3.1 ocio2.1	20m 1s
VFX2019 gcc6/C++14 py2.7 boost1.66 exr2.3 ocio1.1	17m 47s
VFX2020 gcc6/C++14 py3.7 boost1.70 exr2.4 ocio1.1	12m 48s
VFX2021 gcc9/C++17 py3.7 boost1.73 exr2.5 ocio2.0	21m 40s
VFX2021 clang10/C++14 avx2 exr2.5 ocio2.0	16m 41s
VFX2022 gcc9/C++17 py39 boost1.76 exr3.1 ocio2.1	17m 50s
VFX2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	17m 29s
VFX2022 icc/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	20m 16s
VFX2022 icx/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	21m 38s
VFX2022 sanitizers	32m 14s
VFX2023 gcc11/C++17 py3.10 boost1.80 exr3.1 ocio2.2	9m 58s
VFX2019 oldest/hobbled gcc6.3/C++14 py2.7 boost-1.66 exr-2.3 no-sse no-ocio	9m 18s
VFX2022 clang-format	4m 32s
	6h 38m 54s

Provide feedback