The `GZIP` environment variable is [deprecated](https://www.gnu.org/software/gzip/manual/gzip.html#Environment), and everywhere that we invoke `gzip` we are already passing `-9n` directly, i.e:
```base
  find bitcoin-* | sort | tar --no-recursion --mode='u+rw,go+r-w,a+X' --owner=0 --group=0 -c -T - | gzip -9n > ../$SOURCEDIST
```

```bash
GZIP="-9n" gzip -h
gzip: warning: GZIP environment variable is deprecated; use an alias or script
Usage: gzip [OPTION]... [FILE]...
```

Fixes regression introduced by #16141.

```
-BEGIN VERIFY SCRIPT-
sed -i 's#\$REFERENCE_DATE\\\\\\ \$REFERENCE_TIME#\${REFERENCE_DATETIME}#g' contrib/gitian-descriptors/*
-END VERIFY SCRIPT-
```

-----

Note that this could have been fixed by escaping properly, but using `REFERENCE_DATETIME` directly is simpler.

Future note: `REFERENCE_{DATE{,DATETIME},TIME}` is a bit ridiculous. At the very _least_ gitian should use epoch, as it is the most parse-able, and preferably set SOURCE_DATE_EPOCH.