Update CI to buster

.gitlab-ci.yml

@@ -3,7 +3,7 @@ include: "https://git.autistici.org/ai3/build-deb/raw/master/ci-common.yml"
 # debian/patches patches are kept applied in the tree, no need to
 # double-apply them when building the source package.
 # Thus remove the patches altogether and build an original tarball out of HEAD.
-build_pkgsrc:stretch:
+build_pkgsrc:buster:
   script:
     - rm -rf debian/patches
     - git archive --prefix orig/ --output ../$(dpkg-parsechangelog -SSource)_$(dpkg-parsechangelog -SVersion | sed -e  's/-[^-]*$//' -e 's/^[0-9]*://').orig.tar.gz HEAD