Bug fix: glue s3 path

[drobinson-moj]

The current version is broken as var.existing_bucket_name is not always populated.
Corrected the path. Tested as much as I can in nomis-test but the proof of the pudding will be CSR, but I need it merged into main to test that.

[github-actions]

TFSEC Scan Success

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TFSEC will check the following folders:
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Running TFSEC in .
Excluding the following checks: AWS089, AWS099, AWS009, AWS097, AWS018

======================================================
tfsec is joining the Trivy family

tfsec will continue to remain available 
for the time being, although our engineering 
attention will be directed at Trivy going forward.

You can read more here: 
https://github.com/aquasecurity/tfsec/discussions/1994
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  timings
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  disk i/o             264.205µs
  parsing              369.565998ms
  adaptation           491.907µs
  checks               10.599734ms
  total                380.921844ms

  counts
  ──────────────────────────────────────────
  modules downloaded   1
  modules processed    2
  blocks processed     86
  files read           8

  results
  ──────────────────────────────────────────
  passed               10
  ignored              5
  critical             0
  high                 0
  medium               0
  low                  0


No problems detected!

tfsec_exitcode=0

Checkov Scan Success

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Checkov will check the following folders:
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Running Checkov in .
Excluding the following checks: CKV_GIT_1
2023-11-10 15:26:56,899 [MainThread  ] [WARNI]  Failed to download module github.com/ministryofjustice/modernisation-platform-terraform-s3-bucket?ref=8688bc15a08fbf5a4f4eef9b7433c5a417df8df1:None (for external modules, the --download-external-modules flag is required)
terraform scan results:

Passed checks: 61, Failed checks: 0, Skipped checks: 3

github_actions scan results:

Passed checks: 176, Failed checks: 0, Skipped checks: 0


checkov_exitcode=0

CTFLint Scan Success

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Setting default tflint config...
Running tflint --init...
Installing `terraform` plugin...
Installed `terraform` (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.2.1)
tflint will check the following folders:
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Running tflint in .
tflint_exitcode=0