probe: repair the buildctl invocation after a bad sed

.github/workflows/probe-buildkitd.yml

@@ -60,6 +60,5 @@ jobs:
             --frontend dockerfile.v0 \
             --local context="$work" \
             --local dockerfile="$work" \
-            --opt platform=linux/amd64
-            --output type=cacheonly 2>&1 | grep -vE "^#[0-9]+ (sha256|extracting|resolve|DONE|CACHED)" | tail -40
+            --opt platform=linux/amd64 2>&1 | grep -vE "^#[0-9]+ (sha256|extracting|resolve|DONE|CACHED)" | tail -40
           echo "probe finished (results above, from buildkitd's network position)"