cran.loader: Fix parsing description file error

Fix error SWH-LOADER-CORE-1B [1]

UnicodeDecodeError is raised on some DESCRIPTION file. Which makes the loading fail ungraciously.

• [1] https://sentry.softwareheritage.org/share/issue/22e9853f1950442f96b25bdfa6cf09f4/

Test Plan

tox

---

Migrated from D2510 (view on Phabricator)