build: explicitly force .xz compression (old debuild picks gzip) (#19118)

(cherry picked from commit c0b9c763bb)

build/ci.go

@@ -507,7 +507,7 @@ func doDebianSource(cmdline []string) {
 		for _, distro := range debDistros {
 			meta := newDebMetadata(distro, *signer, env, now, pkg.Name, pkg.Version, pkg.Executables)
 			pkgdir := stageDebianSource(*workdir, meta)
-			debuild := exec.Command("debuild", "-S", "-sa", "-us", "-uc", "-d")
+			debuild := exec.Command("debuild", "-S", "-sa", "-us", "-uc", "-d", "-Zxz")
 			debuild.Dir = pkgdir
 			build.MustRun(debuild)