resetEnv() conditionally set POSTGRES_HOST only when empty. When
TestConnect_InvalidCredentials explicitly set it to 'localhost' and
then called resetEnv(), the value was preserved because it wasn't
empty. This leaked into TestConcurrentQueries, which then tried to
connect to localhost:5432 instead of the workflow-configured postgres
hostname.
Fix: capture the POSTGRES_HOST value at init() time in a package-level
variable (savedPOSTGRESHost) and always restore it in resetEnv(), so
the correct value is always used regardless of which tests ran before.
The dev-tagged Connect() in db_dev.go hardcoded localhost:5432 in the
DSN instead of reading the POSTGRES_HOST env var. Since CI tests run
with -tags "test,dev", db_dev.go is compiled and the POSTGRES_HOST=postgres
env var was silently ignored, causing db package tests to try connecting
to 127.0.0.1:5432 where no PostgreSQL is listening (service container
is only reachable via Docker DNS hostname postgres).
Also remove the -a flag from the workflow now that caching is no longer
suspected of causing issues.
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