[viceice]

Changes

• setup: remove the runner's packages.microsoft.com apt sources before apt-get update in the squid-deb-proxy step, so an outage there no longer fails the job. Runner images without them are unaffected.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Drop the runner's Microsoft apt source before updating apt in the setup action #2503
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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• Yes — other (please describe):

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