debian-build: use release tarball from build step

…instead of building a release tarball over and over again.

This has become an issue as meson insists on running tests before creating a
release tarball (which is a good policy).

related to #4086

travis/debian-build.sh

@@ -5,14 +5,10 @@ set -x
 
 DEST=$1
 
-mkdir -p build
 cd build
-../configure
-make echo-version > ../I3_VERSION
-make dist-bzip2
 # unpack dist tarball
 mkdir -p "${DEST}"
-tar xf *.tar.bz2 -C "${DEST}" --strip-components=1
+tar xf meson-dist/*.tar.xz -C "${DEST}" --strip-components=1
 cp -r ../debian "${DEST}"
 sed -i '/^\s*libxcb-xrm-dev/d' deb/ubuntu-*/DIST/debian/control || true
 cd "${DEST}"