Fix(redshift): add missing db_user parameter for IAM auth #3853

Job	Run time
changes	7s
test-dbt-versions (1.3)	1m 43s
test-dbt-versions (1.10)	1m 44s
test-dbt-versions (1.4)	1m 33s
test-dbt-versions (1.8)	1m 41s
test-dbt-versions (1.6)	1m 33s
test-dbt-versions (1.7)	1m 40s
test-dbt-versions (1.5)	1m 37s
test-dbt-versions (1.9)	1m 30s
test-vscode	-1s
test-vscode-e2e	-1s
doc-tests	31s
migration-test	1m 27s
cicd-tests-windows	9m 41s
style-and-cicd-tests (3.9)	23m 50s
style-and-cicd-tests (3.11)	25m 27s
style-and-cicd-tests (3.10)	24m 9s
style-and-cicd-tests (3.13)	21m 12s
style-and-cicd-tests (3.12)	24m 35s
engine-tests-docker (mysql)	1m 39s
engine-tests-docker (spark)	4m 46s
engine-tests-docker (postgres)	2m 24s
engine-tests-docker (risingwave)	2m 33s
engine-tests-docker (starrocks)	2m 31s
engine-tests-docker (clickhouse)	3m 4s
engine-tests-docker (mssql)	1m 52s
engine-tests-docker (duckdb)	1m 35s
engine-tests-docker (trino)	8m 46s
ui-style	0s
ui-test	0s
engine-tests-cloud	0s
	2h 53m 8s

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