ci: increase build watchdog timeout to 2 hours

Increase STALL_WATCHDOG timeout from 65m to 120m to prevent false-positive
kills during long download and layer assembly phases.

Assisted-by: Gemini 3.5 Flash via GitHub Copilot
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: castrojo

.github/workflows/build.yml

@@ -444,7 +444,7 @@ jobs:
           attempt=1
           max_attempts=3
           base_extra="${BST_PODMAN_EXTRA_ARGS:-}"
-          stall_secs=3900
+          stall_secs=7200
           while :; do
             rc=0
             : > /tmp/bst-build-out.txt