fix: guard FORCE_NODE_VERSION in the version error path

Installing on the wrong node version dies with "FORCE_NODE_VERSION: unbound
variable" instead of explaining itself. The script runs under set -u and
this is the one place the variable is read bare; the three other reads all
default it.

The lines it never reaches are the ones telling you the override exists and
naming it, so the people who most need that message are the only ones who
cannot see it.

Verified on Windows 11 with node 22 and code-server 4.135.0: before, the
install ends at the unbound variable; after, it prints the version error and
the override instructions, and exits as intended.

Author: denusklo

ci/build/npm-postinstall.sh

@@ -67,7 +67,7 @@ main() {
 
   if [ "$major_node_version" -ne "${FORCE_NODE_VERSION:-24}" ]; then
     echo "ERROR: code-server currently requires node v24."
-    if [ -n "$FORCE_NODE_VERSION" ]; then
+    if [ -n "${FORCE_NODE_VERSION:-}" ]; then
       echo "However, you have overrided the version check to use v$FORCE_NODE_VERSION."
     fi
     echo "We have detected that you are on node v$major_node_version"