mio 1f74da91f1 testing/mkdotenv: fix checksum
Fix tarball filename and update checksum to match.

```
>>> ERROR: mkdotenv: mkdotenv-0.4.9-r3.tar.gz is missing in checksums
```
2026-01-17 00:13:23 +00:00
..
2026-01-17 00:13:23 +00:00