xx-apk: fix mkdir

[crazy-max]

relates to https://github.com/tonistiigi/xx/actions/runs/12260586308/job/34205579093#step:5:879

> [test-clang test-clang 2/2] RUN --mount=type=cache,target=/pkg-cache,sharing=locked ./test-clang.bats:
43.15 #  in test file test-clang.bats, line 313)
43.15 #   `testHelloCPPLLD' failed
43.15 # rm: can't remove '/etc/llvm/xx-default.cfg': No such file or directory
43.15 # rm: can't remove '/usr/bin/*-apple-*-clang': No such file or directory
43.16 # rm: can't remove '/usr/bin/*-apple-*-clang++': No such file or directory
43.16 # mkdir: can't create directory '/loongarch64-alpine-linux-musl/etc/apk/keys': File exists
44.19 ok 23 386-c++-lld
45.49 ok 24 darwin-setup

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