Add Linux clang build #992

Job	Run time
Linux.1: Release / steps	14m 19s
Linux.3: static / steps	14m 15s
Linux.2: Debug / steps	24m 28s
Linux.4: threads=OFF / steps	13m 44s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / steps	4m 21s
Linux.6: custom namespace / steps	14m 48s
Linux.7: clang / steps	14m 3s
Linux.8: vfx2023 / steps	13m 57s
macOS.1: Release / steps	10m 34s
macOS.2: Debug / steps	20m 54s
macOS.3: static / steps	8m 58s
macOS.4: threads=OFF / steps	9m 21s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 18s
macOS.6: macos-13 / steps	21m 33s
Windows.1: Release / steps	13m 25s
Windows.2: Debug / steps	40m 53s
Windows.3: static / steps	13m 24s
Windows.4: threads=OFF / steps	14m 1s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 33s
Windows.6: vfx2022 / steps	12m 58s
	4h 42m 47s

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