ci: use gRPC-1.26.x to workaround a known bug

[devbww]

Workaround grpc/grpc#21280 by compiling against a specific commit
(without a tag) of the v1.26.x gRPC branch.

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This change is 

[codecov]

Codecov Report

Merging #1232 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1232      +/-   ##
==========================================
- Coverage   94.97%   94.94%   -0.03%     
==========================================
  Files         184      184              
  Lines       13820    13821       +1     
==========================================
- Hits        13125    13122       -3     
- Misses        695      699       +4

Impacted Files	Coverage Δ
...loud/spanner/internal/partial_result_set_resume.cc	90.9% <0%> (-4.55%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-2.23%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
google/cloud/spanner/internal/spanner_stub.cc	69.56% <0%> (-1.09%)	⬇️
google/cloud/spanner/client.cc	96.55% <0%> (-0.87%)	⬇️
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (-0.01%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.36% <0%> (ø)	⬆️
google/cloud/spanner/transaction.h	100% <0%> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	88.04% <0%> (+0.18%)	⬆️
... and 1 more

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[devbww]

It looks like you may have approved the google_cloud_cpp version twice, instead of hitting this one?