xx-apk: fix problem with writing lock in latest alpine

[tonistiigi]

xx-apk can return 0.069 mkdir: can't create directory '': No such file or directory in latest alpine 3.21

xx/src/xx-apk

Line 7 in b0cde9e

mkdir -p "$(readlink -f /var/lock)"

because readlink -f does not return a valid path. This seems to depend on the working directory. I don't see such behavior in coreutils and man readlink also does not suggest this as a valid behavior afaics.

As a workaround, temporarily switch the working directory to root before calling readlink.

[tonistiigi]

https://github.com/mirror/busybox/blob/371fe9f71d445d18be28c82a2a6d82115c8af19d/libbb/xreadlink.c#L160-L161
https://github.com/coreutils/gnulib/blob/03326a89415e32ce1a919336abb8730295b8703e/lib/canonicalize.c#L297-L306

[tonistiigi]

Busybox issue tracked in https://bugs.busybox.net/show_bug.cgi?id=16273