fix: import URL on postMessage script to send slack message on release completed

[CrisTofani]

Short description

Minor fix to restore message on release completed to slack channel

How to test

Latest message was sent from local env

[pagopa-github-bot]

	Warnings
⚠️	Please include a Jira ticket at the beginning of the PR title

Example of PR titles that include Jira tickets:

• single story: [PROJID-123] my PR title
• multiple stories: [PROJID-1,PROJID-2,PROJID-3] my PR title

Generated by 🚫 dangerJS against 4ba4b8b

[shadowsheep1]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.02%. Comparing base (4f204b4) to head (4ba4b8b).
Report is 261 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5952      +/-   ##
==========================================
+ Coverage   48.42%   49.02%   +0.59%     
==========================================
  Files        1488     1714     +226     
  Lines       31617    34297    +2680     
  Branches     7669     8366     +697     
==========================================
+ Hits        15311    16814    +1503     
- Misses      16238    17427    +1189     
+ Partials       68       56      -12     

see 1022 files with indirect coverage changes

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