build-rdeps: Use all available distros by default (Closes: #1032396)

Gioele Barabuccirequested to merge
gioele/devscripts:build-rdeps-not-only-unstable into master

Out of the box build-rdeps errors out when run inside a non-unstable Debian install:

From #1032396:

$ cat /etc/apt/sources.list
deb http://deb.debian.org/debian/ bookworm main
deb-src http://deb.debian.org/debian/ bookworm main

$ build-rdeps --sudo --update systemtap
Hit:1 http://deb.debian.org/debian bookworm InRelease
Reading package lists... Done
build-rdeps: unable to find sources files.
Did you forget to run apt-get update (or add --update to this command)?

It makes more sense for build-rdeps to use whatever Source files are available and restrict itself to unstable/sid only if explicity requested via --only-devel.

Closes: #1032396

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