CI: Fix changed-files action usage

[Turbo87]

Unfortunately this action needs to perform git fetch operations in some cases, which apparently requires persist-credentials to be true.

[Turbo87]

huh... apparently that does not fix it, even though the error message says so 😂

[Turbo87]

fetch-depth: 0 appears to fix it... this makes the checkout action take about 7 seconds, but since we only need to use this once and not for every follow-up job, this seems like a decent tradeoff.