In order to prevent `ngcc`'d packages (e.g. libraries) from getting
accidentally published, `ngcc` overwrites the `prepublishOnly` npm
script to log a warning and exit with an error. In case we want to
restore the original script (e.g. "undo" `ngcc` processing), we keep a
backup of the original `prepublishOnly` script.
Previously, running `ngcc` a second time (e.g. for a different format)
would create a backup of the overwritten `prepublishOnly` script (if
there was originally no `prepublishOnly` script). As a result, if we
ever tried to "undo" `ngcc` processing and restore the original
`prepublishOnly` script, the error-throwing script would be restored
instead.
This commit fixes it by ensuring that we only back up a `prepublishOnly`
script, iff it is not the one we created ourselves (i.e. the
error-throwing one).
PR Close#32427
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