script: re-try apt-get if it fails with "Mirror sync in progress?"

What

Retry apt-get update in case of error.

Why

https://gitlab.com/tezos/tezos/-/jobs/8649249064

Apparently this is not uncommon : https://github.com/search?q=apt-get+update+Mirror+sync+in+progress&ref=opensearch&type=discussions

It seems the common answer is : "Retry later", and this is very much true also for our CI jobs as this error is transient.

How

I override the apt-get command adding a retry of 5 with exponential backoff if I see this error happening.

aliasing the command forced me to drop sudo that it was there only because of documentation purposes.

Manually testing the MR

It's tricky to test if this Mr actually fix the problem. I didn't find a way to trigger the error artificially. The reviewer should carefully inspect the shell script and make sure does not contain any mistakes. Then we can merge and see.

The CI ensure that al least this MR does not affect the tests in case of no errors.

This is s scheduled pipeline : https://gitlab.com/nomadic-labs/tezos/-/pipelines/1615908664

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR