fix: authn: allow builtin credential handling to fail conditionally (… #1685

Job	Run time
matrix-prep-bazelversion	2s
matrix-prep-os	1s
test (ubuntu-latest, 7.4.1, ., true)	4m 43s
test (ubuntu-latest, 7.4.1, e2e/wasm, false)	2m 0s
test (ubuntu-latest, 7.4.1, e2e/smoke, true)	13s
test (ubuntu-latest, 7.4.1, e2e/smoke, false)	14s
test (ubuntu-latest, 7.4.1, e2e/assertion, true)	1m 33s
test (ubuntu-latest, 6.4.0, e2e/wasm, false)	27s
test (ubuntu-latest, 6.4.0, e2e/smoke, true)	12s
test (ubuntu-latest, 6.4.0, e2e/smoke, false)	13s
test (ubuntu-latest, 6.4.0, e2e/assertion, true)	1m 42s
test (macos-13, 7.4.1, ., true)	8m 8s
test (macos-13, 7.4.1, e2e/smoke, true)	1m 35s
test (macos-13, 7.4.1, e2e/smoke, false)	1m 52s
	22m 55s

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