No pcaps when platforms don't match

[larseggert]

When I run a QNS test locally on my ARM macBook that involves an image that isn't available as linux/arm64/v8 but only as linux/amd64, the test runs (output.txt has content) but then no pcaps are copied over from the simulator. And that then often causes the test to fail.

Example: python -u run.py -c neqo -s nginx -t handshake

[larseggert]

Will likely be fixed by quic-interop/quic-network-simulator#133

[nhorman]

FWIW, i'm seeing a similar problem. Trying to run interop locally with an openssl container and any number of servers, and randomly it appears that one or both of the trace files is empty. Going to try with quic-interop/quic-network-simulator#133

[tatsuhiro-t]

As I wrote in #403, saw this issue in regular interop runner run.