common-updater-scripts: Use custom fake temporary hash

Some update scripts (e.g. sublime4) already use all-zeroes hashes
for resetting a version when updating multiple sources.
When `update-source-version "${attrPath}" 0 "${lib.fakeSha256}"`
was executed, u-s-v would not be able to detect that the hash
changed and would fail with the following message:

    Failed to replace temporary source hash of '…' to the final source hash!

The update script worked when we tested it because change detection itself
was broken until recently: https://github.com/NixOS/nixpkgs/pull/190554

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