Don't retun err if go.mod exists

[Madrigal]

Issue #, if available:

Description of changes:
We made a change on aws/smithy-go#558 that changes the default for a Go project to generate a go.mod file by default. This broke the existing workflow on aws/aws-sdk-go-v2 since generate now fails with

+ go run github.com/awslabs/aws-go-multi-module-repository-tools/cmd/gomodgen@latest -build /Volumes/workplace/go-sdk/aws-sdk-go-v2/codegen/protocol-test-codegen/build/smithyprojections/protocol-test-codegen
2024/12/18 15:01:52 failed to copy build artifacts: failed to copy build artifact to target: open /Volumes/workplace/go-sdk/aws-sdk-go-v2/internal/protocoltest/ec2query/go.mod: file exists

This changes the line where we open the file to overwrite any existing content.

Testing

Setup

I created a fork of the repo since I wasn't able to get it to run locally. I then modified the copy codegen script to have the following

#!/usr/bin/env bash

set -xe

SDK_ROOT=$1
CODEGEN_ROOT=$2

REPOTOOLS_VERSION="fix-overwrite-existing-gomodo"

cd "$1"
go run github.com/Madrigal/aws-go-multi-module-repository-tools/cmd/gomodgen@${REPOTOOLS_VERSION} -build "$CODEGEN_ROOT"

And ran make. Ensure clients get generated and tests pass successfully, with no meaningful diff.

The main thing to consider is that all files now have this diff

diff --git a/service/amplify/generated.json b/service/amplify/generated.json
index cc33468608..ab676f0a0d 100644
--- a/service/amplify/generated.json
+++ b/service/amplify/generated.json
@@ -52,6 +52,7 @@
         "endpoints_config_test.go",
         "endpoints_test.go",
         "generated.json",
+        "go.mod",
         "internal/endpoints/endpoints.go",
         "internal/endpoints/endpoints_test.go",
         "options.go",
@@ -63,7 +64,7 @@
         "types/types.go",
         "validators.go"
     ],
-    "go": "1.15",
+    "go": "1.21",
     "module": "github.com/aws/aws-sdk-go-v2/service/amplify",
     "unstable": false
 }

Which I suspect should be fine since I think these files are just informative, but someone correct me if I'm wrong

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