package_manager.py: Fix race condition in OpkgIndexer.write_index()

package_manager.py: Fix race condition in OpkgIndexer.write_index()

When writing the index using ipk packages there could be a race condition
when populate the index. This happens because the architectures
are repeated (specially all) and the commands generated to write the index
run in parallel.

This change avoid the duplication of commands using a set instead of a list.

[YOCTO #8924]

(From OE-Core rev: 74adb14b0002e20099cc2c34e01862e8ddb8e013)

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>