fix(perf): update df_engine mount path to /home/nonroot in syslog-tcp-otelcol nightly suite

[cijothomas]

The nightly Pipeline Performance Tests run 26069747923 failed in the "Run syslog TCP performance test otelcol log suite" step because the backend (df_engine) container never became ready (readiness check timed out after 10 attempts).

Root cause: PR #2919 switched the df_engine image from musl/alpine to glibc/distroless, changing the in-container home directory from /home/dataflow to /home/nonroot, and updated the volume mount path in all then-existing nightly docker yamls. The syslog-tcp-otelcol-docker.yaml suite (added concurrently in #2962) was missed, so it was still mounting the backend config at /home/dataflow/config.yaml — a path that doesn't exist in the new image — causing the backend container to fail to start.

This one-line change brings it in line with the other nightly suites.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.95%. Comparing base (2688ea1) to head (75cc246).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3032      +/-   ##
==========================================
- Coverage   85.95%   85.95%   -0.01%     
==========================================
  Files         725      725              
  Lines      276289   276289              
==========================================
- Hits       237485   237474      -11     
- Misses      38280    38291      +11     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	87.08% <ø> (-0.01%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	89.57% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

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