Ports: Use sed -i for run_replace_in_file on Serenity hosts

We don't have a perl port, so let's use GNU sed's in-place option for
doing substitutions when running inside Serenity.

Note that we can't do this unconditionally, as `sed -i` is not portable
and works differently on BSD systems.

Ports/.port_include.sh

@@ -108,7 +108,11 @@ run() {
 }
 
 run_replace_in_file() {
-    run perl -p -i -e "$1" $2
+    if [ "$(uname -s)" = "SerenityOS" ]; then
+        run sed -i "$1" $2
+    else
+        run perl -p -i -e "$1" $2
+    fi
 }
 
 get_new_config_sub() {