ci: Simplify build_llvm.bash script (#3892) #4108

Job	Run time
VFX2019 gcc6/C++14 py2.7 boost1.66 exr2.3 ocio1.1	15m 11s
VFX2020 gcc6/C++14 py3.7 boost1.70 exr2.4 ocio1.1	11m 41s
VFX2021 gcc9/C++17 py3.7 boost1.73 exr2.5 ocio2.0	18m 20s
VFX2021 clang10/C++14 avx2 exr2.5 ocio2.0	16m 59s
VFX2022 gcc9/C++17 py39 boost1.76 exr3.1 ocio2.1	19m 36s
VFX2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	16m 20s
VFX2022 icc/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	24m 22s
VFX2022 icx/C++17 py3.9 boost1.76 exr3.1 ocio2.1 qt5.15	21m 1s
VFX2022 sanitizers	41m 38s
VFX2023 gcc11/C++17 py3.10 boost1.80 exr3.1 ocio2.2	17m 53s
VFX2019 oldest/hobbled gcc6.3/C++14 py2.7 boost-1.66 exr-2.3 no-sse no-ocio	16m 20s
VFX2022 clang-format	4m 10s
Ubuntu latest releases gcc11 C++17 avx2 exr3.1 ocio2.2	22m 28s
Ubuntu bleeding edge gcc12 C++20 py3.10 OCIO/libtiff/exr-master boost1.74 avx2	25m 4s
Ubuntu clang14 C++20 avx2 exr3.1 ocio2.1	27m 45s
Ubuntu debug gcc7/C++14, sse4.2, exr2.4	21m 1s
Ubuntu gcc8 C++17 avx exr2.5	14m 56s
Ubuntu static libs gcc7 C++14 exr2.4	19m 29s
windows-2019 VS2019	37m 36s
macos-11 appleclang13/C++17 py3.9 MacOS-11 boost1.76 exr3.1 ocio2.1	26m 56s
macos-12 appleclang13/C++20 py3.10 MacOS-12 boost1.76 exr3.1 ocio2.1	40m 5s
macos-13 appleclang14/C++20 py3.11 MacOS-13 boost1.76 exr3.1 ocio2.1	19m 12s
	7h 58m 3s

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