Restructure CI to use reusable steps, and add new checks #965

Job	Run time
macOS.2: Debug / ci_steps	19m 48s
macOS.1: Release / ci_steps	7m 58s
macOS.3: static / ci_steps	8m 1s
macOS.4: threads=OFF / ci_steps	7m 17s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / ci_steps	1m 9s
macOS.6: macos-13 / ci_steps	19m 35s
Linux.1: Release / ci_steps	15m 59s
Windows.1: Release / ci_steps	13m 9s
Windows.2: Debug / ci_steps	2h 39m 0s
Windows.3: static / ci_steps	13m 4s
Windows.4: threads=OFF / ci_steps	13m 11s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / ci_steps	2m 10s
Windows.6: vfx2022 / ci_steps	12m 52s
Linux.2: Debug / ci_steps	24m 9s
Linux.3: static / ci_steps	14m 11s
Linux.6: custom namespace / ci_steps	14m 15s
Linux.4: threads=OFF / ci_steps	13m 29s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / ci_steps	4m 13s
Linux.7: vfx2023 / ci_steps	13m 44s
Linux.8: vfx2022 / ci_steps	13m 20s
Linux.9: vfx2021 / ci_steps	13m 25s
	6h 43m 59s

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