CI: don't upload test coverage multiple times

[rbino]

Pick a single Postgres version in the matrix and use it to upload coverage. Other versions just run tests.

[coveralls]

Pull Request Test Coverage Report for Build 01af5232a8224e1157ffd6e5a94cb0f240517ea8-PR-680

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+80.6%) to 80.565%

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Totals
Change from base Build 11557683634:	80.6%
Covered Lines:	1227
Relevant Lines:	1523

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💛 - Coveralls