reprotest: doesn't respect $RELEASE

I configured xdg-desktop-portal-gtk with RELEASE: experimental because the debian/master branch has build-dependencies that aren't in unstable yet.

It looks as though the reprotest job doesn't respect this:

https://salsa.debian.org/debian/xdg-desktop-portal-gtk/-/jobs/202379

$ apt-get update
Get:1 http://cdn-fastly.deb.debian.org/debian unstable InRelease [210 kB]
Get:2 http://cdn-fastly.deb.debian.org/debian unstable/main amd64 Packages [8307 kB]
Fetched 8517 kB in 2s (3912 kB/s)
...
Reading package lists...
$ eatmydata apt-get build-dep -y .
Note, using directory '.' to get the build dependencies
...
The following packages have unmet dependencies:
 builddeps:. : Depends: xdg-desktop-portal-dev (>= 1.4) but it is not going to be installed