scripts: run-crossversion-meta: use artifacts subdir

Currently crossversion artifacts go directly inside the crossversion
directory. They end up being gitignored (because they contain .test)
but they are hard to clean up and can't easily be configured to be
ignored by IDEs.

We now put everything in an `artifacts` subdir (already gitignored).

Author: RaduBerinde

scripts/run-crossversion-meta.sh

@@ -37,6 +37,7 @@ if [[ -z "${STRESS}" ]]; then
       -test.run 'TestMetaCrossVersion$' \
       -seed ${SEED:-0} \
       -factor ${FACTOR:-10} \
+      -artifacts ./artifacts \
       $(echo $VERSIONS)
 else
     stress -p 1 go test ./internal/metamorphic/crossversion \
@@ -45,6 +46,7 @@ else
       -test.run 'TestMetaCrossVersion$' \
       -seed ${SEED:-0} \
       -factor ${FACTOR:-10} \
+      -artifacts ./artifacts \
       $(echo $VERSIONS)
 fi