__debian_backports: pass shellcheck for sourced file

Because the sourced explorer can't be detected by shellcheck, it will be
completely disabled. Changing the path to /etc/os-release isn't
deterministic either.

The shellcheck wiki page suggests to use `source=/dev/null` instead of
`disable=SC1090`, but it was choosen to completely avoid that check ..

cdist/conf/type/__debian_backports/manifest

@@ -30,6 +30,7 @@ case "$os" in
         # distribution codename from /etc/os-release
         # lsb_release may not be given in all debian installations
         dist="$(
+            # shellcheck disable=SC1090
             . "$__global/explorer/os-release"
             printf "%s" "$VERSION_CODENAME"
         )"