cacert: fix certdata2pem url

Related to #39927

```
$ nix-prefetch-url https://salsa.debian.org/debian/ca-certificates/raw/debian/20170717/mozilla/certdata2pem.py
path is '/nix/store/0d00axdac4h8ffxrf90s5zh8xdw3r29z-certdata2pem.py'
1d4q27j1gss0186a5m8bs5dk786w07ccyq0qi6xmd2zr1a8q16wy
```