build: noop clean during travis debsrc assembly step

build/clean_go_build_cache.sh

@@ -1,5 +1,12 @@
 #!/bin/sh
 
+# Cleaning the Go cache only makes sense if we actually have Go installed... or
+# if Go is actually callable. This does not hold true during deb packaging, so
+# we need an explicit check to avoid build failures.
+if ! command -v go > /dev/null; then
+  exit
+fi
+
 version_gt() {
   test "$(printf '%s\n' "$@" | sort -V | head -n 1)" != "$1"
 }