Add Linux clang build #983

Job	Run time
Linux.3: static / steps	14m 41s
Linux.2: Debug / steps	24m 51s
Linux.10: vfx2021 / steps	13m 17s
Linux.1: Release / steps	14m 23s
Linux.4: threads=OFF / steps	13m 24s
Linux.6: custom namespace / steps	14m 8s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / steps	4m 20s
Linux.7: clang / steps	13m 58s
Linux.8: vfx2023 / steps	13m 56s
Linux.9: vfx2022 / steps	13m 22s
macOS.2: Debug / steps	19m 24s
macOS.1: Release / steps	7m 26s
macOS.3: static / steps	9m 28s
macOS.4: threads=OFF / steps	7m 1s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 7s
macOS.6: macos-13 / steps	26m 58s
Windows.1: Release / steps	13m 14s
Windows.2: Debug / steps	39m 43s
Windows.3: static / steps	12m 23s
Windows.4: threads=OFF / steps	13m 16s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 53s
Windows.6: vfx2022 / steps	13m 30s
	5h 5m 43s

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