WIP: github: Only run the ffmpeg test jobs #1501

Job	Run time
prepare	0s
test-ffmpeg	18m 37s
linux-test-cross-build-ffmpeg (i686)	5m 35s
linux-test-cross-build-ffmpeg (x86_64)	7m 24s
linux-test-cross-build-ffmpeg (armv7)	4m 43s
linux-test-cross-build-ffmpeg (aarch64)	5m 16s
linux	0s
linux-asserts	0s
macos	0s
msys2	0s
linux-cross-aarch64	0s
linux-cross-windows	0s
test-openmp	0s
test-libcxx	0s
test-toolchain	0s
test-compiler-rt	0s
upload-nightly	0s
	41m 35s

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