drop executable requirement for INTEGRATION_TEST_CONFIG

[mr-salty]

we only source that script so it doesn't need to be executable, only
readable.

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This change is 

[codecov]

Codecov Report

Merging #1099 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1099      +/-   ##
==========================================
- Coverage   92.99%   92.83%   -0.16%     
==========================================
  Files         164      164              
  Lines       12151    12149       -2     
==========================================
- Hits        11300    11279      -21     
- Misses        851      870      +19

Impacted Files	Coverage Δ
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	69.9% <0%> (-4.16%)	⬇️
google/cloud/spanner/internal/spanner_stub.cc	68.13% <0%> (-1.1%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	82.45% <0%> (-0.88%)	⬇️
google/cloud/spanner/value.h	92.7% <0%> (-0.11%)	⬇️
google/cloud/spanner/instance_admin_connection.h
google/cloud/spanner/database_admin_connection.h	100% <0%> (ø)
google/cloud/spanner/samples/samples.cc	77.85% <0%> (+0.19%)	⬆️
google/cloud/spanner/internal/polling_loop.h	94.59% <0%> (+2.7%)	⬆️
google/cloud/spanner/internal/retry_loop.h	100% <0%> (+4.54%)	⬆️

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