fix: Write os-var replaced .src file to temp file, and rename

The replace_os_vars function is called for any command, not only those that boot the release. If you invoke <release> status in a readiness check, that will write the file again.

This might lead to race conditions, where the release starts up, but the health check will re-render the sys.config and write it halfway while the boot gets around to read it.

priv/templates/extended_bin

@@ -445,6 +445,7 @@ make_out_file_path() {
 
 # Replace environment variables
 replace_os_vars() {
+    TEMP_OUT=$(mktemp "$2".XXXXX)
     awk '{
         while(match($0,"[$]{[^}]*}")) {
             var=substr($0,RSTART+2,RLENGTH -3)
@@ -460,7 +461,8 @@ replace_os_vars() {
                 gsub("[$]{"var"}",e)
             }
         }
-    }1' < "$1" > "$2"
+    }1' < "$1" >> "$TEMP_OUT"
+    mv "$TEMP_OUT" "$2"
 }
 
 add_path() {