fix(get-fileshare-signed-url) ensure the script is re-entrant to allow concurent calls

[dduportal]

Ref. https://github.com/jenkins-infra/helpdesk/issues/2649

This PR fixes the shell script resources/get-fileshare-signed-url.sh so it is re-entrant to support concurrent calls as part of https://github.com/jenkins-infra/update-center2/pull/777.

The documentation at https://learn.microsoft.com/en-us/cli/azure/use-azure-cli-successfully?tabs=bash%2Cbash2#concurrent-execution shows that we can use a different configuration dir. on each call

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Validations:

• Shellcheck (of course)
• Ran the script on my local development machine (smoke test: if it smokes then it is bad ;))
• On the trusted.ci permanent agent:
  • Disabled the puppet agent (to avoid overriding changes)
  • Added the change
  • Waited for 2 consecutive successful builds of update_center2 on trusted.ci