ci: Simplify rules for collecting build artifacts

The script `ci/build_examples.sh` only copies files that are necessary
for running examples into the destination folders, so we can simply take
the whole folders as artifacts.

Compared to hardcoding a list of artifacts, this simplification also
adds flexibility in terms of what to put in the destination folder. For
example, `update-signature.txt` is specific to AWS OTA and pointless in
all other cases.

Also rename the variable `BUILD_FOLDER` to `OUTPUT_FOLDER`, because it
is not actually where the build is done, but where artifacts required
for testing are copied into by `ci/build_examples.sh`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>