imagebuilder: skip repository file when standalone

imagebuilder: skip repository file when standalone

Standalone image builder doesn't have a repositories file as all
packages are included, which causes:

ERROR: failed to read repositories: PATH_TO_BUILDER/repositories: No such file or directory

The images are still built, so this is more of an informational error.

Pass related argument to apk only when CONFIG_IB_STANDALONE is not set.

Fixes: a8d17c21 ("imagebuilder: actually support IB from buildbot")
Signed-off-by: George Sapkin <george@sapk.in>
Link: https://github.com/openwrt/openwrt/pull/21658
Signed-off-by: Robert Marko <robimarko@gmail.com>