ci: workaround a bug in gRPC < 1.15.0

[coryan]

Before 1.25.0 gRPC sometimes crashes when using the c-ares DNS resolver
on Windows, see grpc/grpc#18461 for more details.

I think this will minimize the problems reported in #1034

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This change is 

[codecov]

Codecov Report

Merging #1158 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
- Coverage   95.76%   95.74%   -0.03%     
==========================================
  Files         170      170              
  Lines       13159    13163       +4     
==========================================
+ Hits        12602    12603       +1     
- Misses        557      560       +3

Impacted Files	Coverage Δ
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (-2.07%)	⬇️
google/cloud/spanner/client.cc	96.55% <0%> (-0.87%)	⬇️
google/cloud/spanner/samples/samples.cc	86.37% <0%> (ø)	⬆️
google/cloud/spanner/internal/time_format.cc	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.87% <0%> (ø)	⬆️
google/cloud/spanner/value.h	92.8% <0%> (+0.1%)	⬆️

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