use retryablehttp in DiscoverKeys and pkg/apk #2513

Job	Run time
annotations	1m 6s
build-all-examples-amd64 (ubuntu-latest)	1m 41s
build-all-examples-amd64 (macos-latest)	1m 40s
build-date-epoch	1m 36s
source-date-epoch	1m 41s
build-nginx-multiarch	1m 7s
build-nginx-all-arches (x86_64)	1m 6s
build-nginx-all-arches (386)	1m 4s
build-nginx-all-arches (armv7)	1m 4s
build-nginx-all-arches (aarch64)	1m 13s
build-nginx-all-arches (riscv64)	1m 3s
build-nginx-all-arches (s390x)	1m 5s
build-nginx-all-arches (ppc64le)	1m 6s
	16m 32s

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