feat: change quickstart to use recommended build target

[devjgm]

This change is 

[codecov]

Codecov Report

Merging #1436 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1436      +/-   ##
==========================================
- Coverage   93.55%   93.54%   -0.02%     
==========================================
  Files         191      191              
  Lines       15854    15858       +4     
==========================================
+ Hits        14833    14835       +2     
- Misses       1021     1023       +2

Impacted Files	Coverage Δ
google/cloud/spanner/internal/log_wrapper.h	71.42% <0%> (-13.19%)	⬇️
google/cloud/spanner/samples/samples.cc	82.32% <0%> (-0.21%)	⬇️
google/cloud/spanner/value.h	92.19% <0%> (-0.11%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
...cloud/spanner/internal/partial_result_set_resume.h	100% <0%> (ø)	⬆️
google/cloud/spanner/session_pool_options.h	100% <0%> (ø)	⬆️
...oogle/cloud/spanner/internal/instance_admin_stub.h
...oogle/cloud/spanner/internal/database_admin_stub.h	100% <0%> (ø)
...anner/integration_tests/client_integration_test.cc	98.27% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
... and 1 more

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