ci: enable ccache for the clang-3.8 build

[coryan]

Fixes googleapis/google-cloud-cpp#3610

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This change is 

[codecov]

Codecov Report

Merging #1473 into master will increase coverage by 0.74%.
The diff coverage is 100.00%.

@@ Coverage Diff @@ ## master #1473 +/- ## ========================================== + Coverage 94.85% 95.59% +0.74%  ========================================== Files 194 192 -2 Lines 15963 15915 -48 ========================================== + Hits 15141 15214 +73  + Misses 822 701 -121 

Impacted Files	Coverage Δ
google/cloud/spanner/client_test.cc	94.66% <100.00%> (+0.12%)	⬆️
...le/cloud/spanner/database_admin_connection_test.cc	99.72% <100.00%> (+<0.01%)	⬆️
...le/cloud/spanner/instance_admin_connection_test.cc	99.47% <100.00%> (+<0.01%)	⬆️
...gle/cloud/spanner/internal/connection_impl_test.cc	98.01% <100.00%> (+0.04%)	⬆️
google/cloud/spanner/internal/log_wrapper_test.cc	100.00% <100.00%> (ø)
...spanner/internal/partial_result_set_resume_test.cc	100.00% <100.00%> (ø)
...spanner/internal/partial_result_set_source_test.cc	100.00% <100.00%> (ø)
google/cloud/spanner/keys_test.cc	93.06% <100.00%> (+0.06%)	⬆️
google/cloud/spanner/mutations_test.cc	100.00% <100.00%> (ø)
google/cloud/spanner/results_test.cc	93.75% <100.00%> (+0.29%)	⬆️
... and 20 more

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[devjgm]

I swear, I tried this earlier and couldn't get it to work... glad it's working now. Not sure what I did wrong, but thanks.