Bump external/bpf_conformance from `b544539` to `28ddd7f` #162

Job	Run time
build_ubuntu (Debug, tests)	32m 51s
build_windows (Debug, tests)	45m 0s
build_windows (Debug, library)	5m 28s
build_ubuntu (Debug, library)	1m 26s
build_windows (Release, tests)	10m 4s
build_ubuntu (Release, tests)	6m 20s
build_windows (Release, library)	5m 29s
build_ubuntu (Release, library)	1m 11s
	1h 47m 49s

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