Fix network-limited benchmarks not running

[greschd]

The benchmarks with specific network latency or bandwidth had failed
to run since the grpc_server fixture was renamed to acp_instance,
but the fixture which overrides it in the benchmarks was not.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.29%. Comparing base (0dcd834) to head (2c3ed89).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #627   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files          87       87           
  Lines        4698     4698           
=======================================
  Hits         4383     4383           
  Misses        315      315           

Flag	Coverage Δ
python-3.10	93.16% <ø> (ø)
python-3.11	93.08% <ø> (ø)
python-3.12	93.20% <ø> (ø)
server-2024R2	92.25% <ø> (ø)
server-2025R1	93.08% <ø> (ø)
server-latest	93.16% <ø> (ø)

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[greschd]

There appears to be an error when running the 100ms benchmark; I will need to investigate.

@roosre if this is blocking, feel free to revert the PR.