The previous find invocation didn't match the root directory, so the
root directory's access and modification time wasn't set to a
deterministic value and the build time leaked into the output.
`make-initrd` replaced `cpio` with `bsdtar` in #165892 because the
former includes the number of hardlinks in the created archive, which
depends on the filesystem (and can also be influenced by `nix-store
--optimise`). The same problem applies to `make-initrd-ng`, so this
commit replaces `cpio` with `libarchive`'s `bsdtar`.
Now the tool will only strip binaries if a strip executable is passed
via the STRIP environment variable. This is exposed via the strip
option for makeInitrdNG and the NixOS option boot.initrd.systemd.strip.
