fix: issue with iam variable as list

If the value of `var.secret_accessors` is "unknown until apply" the `for_each`
loop referencing the variable errors. This is because it uses `toset` causing the key
value to also be "unknown until apply", which is not allowed. This commit fixes
this by switching the variable type to `map(string)` to ensure that the keys in
the `for_each` are static.