fix: use absolute path to lambda.zip

filebase64sha256 is unable to find "lib/lambda.zip" when using this
module from another module because the relative path doesn't resolve
to the right place in that context.

This changes both references to lambda.zip to use the absolute path by
prefixing it with `path.module`.

modules/custom_identity_provider/main.tf

@@ -51,8 +51,8 @@ resource "aws_lambda_function" "this_lambda" {
   runtime          = "python3.9"
   description      = "A function to lookup and return user data from AWS Secrets Manager."
   tags             = var.tags
-  filename         = "lib/lambda.zip"
-  source_code_hash = filebase64sha256("lib/lambda.zip")
+  filename         = "${path.module}/lib/lambda.zip"
+  source_code_hash = filebase64sha256("${path.module}/lib/lambda.zip")
 
   environment {
     variables = {