Bump cloud.google.com/go/iam from 1.2.0 to 1.2.1 #3554

Job	Run time
lint	13s
staticcheck	1m 15s
build	43s
tests (macos/go-1.22-amd64)	1m 18s
tests (ubuntu/go-1.22-386)	56s
tests (ubuntu/go-1.22-amd64)	59s
tests (windows/go-1.22-386)	2m 38s
tests (windows/go-1.22-amd64)	3m 20s
tests (macos/go-1.23-amd64)	1m 38s
tests (ubuntu/go-1.23-386)	55s
tests (ubuntu/go-1.23-amd64)	1m 10s
tests (windows/go-1.23-386)	2m 30s
tests (windows/go-1.23-amd64)	3m 7s
test-python-example	15s
test-gsutil-example	28s
test-node-example	15s
test-go-example	32s
test-dotnet-example	23s
test-scala-example	38s
test-java-example	22s
test-cpp-example	2m 15s
test-curl-example	8s
test-curl-both-scheme-default-ports-example	6s
test-curl-both-scheme-non-default-ports-example	6s
unblock-pr	1s
	26m 11s

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