ci: Refactor cached disk image selection logic

[upbqdn]

Motivation

Resolve #8908 (comment).

Solution

• Try other locations if $PREFER_MAIN_CACHED_STATE is true and find_cached_disk_image doesn't return anything.

Tests

No tests.

PR Author's Checklist

• The PR name is suitable for inclusion in the changelog.
• The solution is tested.
• The PR has a priority label.

[gustavovalverde]

Optional code improvement

[gustavovalverde]

To avoid repeating the if condition several times, we can also do:

# Try to find an image based on the `main` branch if that branch is preferred.
if [[ "${PREFER_MAIN_CACHED_STATE}" == "true" ]]; then
    CACHED_DISK_NAME=$(find_cached_disk_image "main-[0-9a-f]+" "main branch")
fi
# If no image was found, try to find one from the current branch (or PR).
CACHED_DISK_NAME=${CACHED_DISK_NAME:-$(find_cached_disk_image ".+-${GITHUB_REF}" "branch")}
# If we still have no image, try to find one from any branch.
CACHED_DISK_NAME=${CACHED_DISK_NAME:-$(find_cached_disk_image ".+-[0-9a-f]+" "any branch")}

# Handle the case where no suitable disk image is found
if [[ -z "${CACHED_DISK_NAME}" ]]; then
    echo "No suitable cached state disk available. Try running the cached state rebuild job."
    exit 1
else
    echo "Selected Disk: ${CACHED_DISK_NAME}"
fi

Here we'd use ${VAR:-value} to set CACHED_DISK_NAME to the result of the next find_cached_disk_image call only if it's not already set.

[gustavovalverde]

Solved within #8908