runvm-osbuild: log disk usage during run

At least temporarily let's output some logging about how much
of the filesystem under cache/ we are using so we can see if
we're getting close to limits and also maybe see which stages
are taking up the most space.

src/runvm-osbuild

@@ -32,6 +32,17 @@ getconfig_def() {
     jq -re .\""$k"\"//\""${default}"\" < "${config}"
 }
 
+log_disk_usage(){
+    # spawn off a subshell in the background to log disk usage every
+    # 10 seconds.
+    yellow="\033[33m"; default="\033[39m"
+    (while true; do
+        echo -e "$yellow"; df -kh ./cache; echo -e "$default";
+        sleep 10;
+        pgrep --exact osbuild >/dev/null || break;
+    done) &
+}
+
 while [ $# -gt 0 ];
 do
     flag="${1}"; shift;
@@ -112,6 +123,8 @@ set -x; osbuild-mpp                                       \
     "${mppyaml}" "${processed_json}"
 set +x
 
+log_disk_usage
+
 # Build the image
 set -x
 # shellcheck disable=SC2068