fix(scripts): Fix awk env parsing for POSIX compatibility

- Use index() instead of regex capture groups for key extraction
- More portable across different awk implementations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/setup.sh

@@ -1478,16 +1478,18 @@ write_env_with_overrides() {
             while ((getline < overrides_file) > 0) {
                 line = $0
                 if (line ~ /^[A-Za-z0-9_]+=*/) {
-                    key = substr(line, 1, index(line, "=") - 1)
+                    eq = index(line, "=")
-                    value = substr(line, index(line, "=") + 1)
+                    key = substr(line, 1, eq - 1)
+                    value = substr(line, eq + 1)
                     map[key] = value
                 }
             }
             close(overrides_file)
         }
         {
-            if (match($0, /^([A-Za-z0-9_]+)=/, m)) {
+            if ($0 ~ /^[A-Za-z0-9_]+=/) {
-                key = m[1]
+                eq = index($0, "=")
+                key = substr($0, 1, eq - 1)
                 if (key in map) {
                     print key "=" map[key]
                     used[key] = 1