python/config: make download_reactively race-safe under parallel exec… #3596

Job	Run time
libvmaf-build (ubuntu-latest, ccache gcc, ccache g++)	8m 45s
libvmaf-build (ubuntu-24.04-arm, ccache clang, ccache clang++)	35s
libvmaf-build (ubuntu-latest, ccache gcc, ccache g++, ./libvmaf/test/meson-toolchains/i686-pc-lin...	10m 12s
libvmaf-build (macos-latest, ccache clang, ccache clang++, true)	10m 50s
libvmaf-build (ubuntu-latest, ccache gcc-9, ccache g++-9)	8m 29s
libvmaf-build (ubuntu-latest, ccache clang, ccache clang++)	8m 58s
libvmaf-build (ubuntu-latest, ccache clang, ccache clang++, ./libvmaf/test/meson-toolchains/i686-...	11m 58s
libvmaf-build (ubuntu-latest, ccache clang, ccache clang++, ./libvmaf/test/meson-toolchains/i686-...	9m 29s
libvmaf-build (ubuntu-latest, ccache gcc, ccache g++, ./libvmaf/test/meson-toolchains/i686-pc-lin...	14m 4s
libvmaf-build (ubuntu-latest, ccache gcc, ccache g++)	8m 45s
libvmaf-build (macos-latest, ccache clang, ccache clang++, true)	10m 50s
libvmaf-build (ubuntu-latest, ccache gcc-9, ccache g++-9)	8m 29s
libvmaf-build (ubuntu-24.04-arm, ccache clang, ccache clang++)	35s
libvmaf-build (ubuntu-latest, ccache clang, ccache clang++)	8m 58s
	2h 0m 57s

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