Problem: Installing plugins during lockfile synchronization always
writes the lockfile, even though its content is used during install.
This might be a problem if the lockfile (itself or its parent
directory) is not writeable (can only be read).
Solution: Do not write the lockfile when installing directly from it.
This is okay since the `src` and `rev` are used directly from the
lockfile and don't change at this step. While potential change in
`version` (that must be written to the lockfile) is handled in other
code path.
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