probe: drop type=cacheonly, this buildkit has no such exporter

buildctl reported: exporter "cacheonly" could not be found. Omitting --output
still runs every RUN step, which is all the probe needs. Same defect exists in
build-image-gitea.yml (validation path and toolchain assertion) and needs its
own fix.

.github/workflows/probe-buildkitd.yml

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