Fix build hangs: add apt-get network timeout and job timeout

apt-get has no default network timeout, so an unresponsive Debian mirror
can block a build stage indefinitely. Add Acquire::http::Timeout=30 to
both update and install calls in all apt-get invocations so mirror hangs
fail fast rather than running until GitHub's 6-hour job limit.

Also add timeout-minutes: 60 to the build job so a runaway step fails
within an hour rather than silently consuming the full 6-hour default.

.github/workflows/build.yaml

@@ -75,6 +75,7 @@ jobs:
 
   build:
     runs-on: ubuntu-latest
+    timeout-minutes: 60
     if: (github.event_name == 'push' || github.event_name == 'workflow_dispatch') && github.ref == 'refs/heads/master'
     strategy:
       fail-fast: false