fix(homebrew): always extract the URL and sha256 from current filecontent

[secustor]

Changes

Extract the current URL and sha256 again before replacement so that the replacement will be correct even if the branch has been updated in the meantime.

Context

If a PR has been opened and then a new release would be published, then the file content would not be updated.
That is as the managerdata would contain the orginal values and NOT the intermediately updated ones

Please select one of the following:

• This closes an existing Issue, Closes: #
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The public repository: secustor/renovate-homebrew-repro#4