Don't “fix” the permissions of preexisting DOT_SAGE

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See the commit message for details.

Component: **scripts**

Author: **Marc Mezzarobba**

Branch/Commit: **[`387fdfc`](https://github.com/sagemath/sagetrac-mirror/commit/387fdfc5528b154ac99ce75ef4facb7d903c1dc4)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/29093_

