feat(burnin): add build.sh for Linux guest + Start-BurnInTest-Linux.ps1

gitea/burnin-dummy/build.sh: bash equivalent of build.ps1 (sleep 30,
writes dist/build-info.txt). Needed because powershell is not present
on LinuxBuild2404 template.

scripts/Start-BurnInTest-Linux.ps1: burn-in wrapper for LinuxBuild2404
template (SnapshotName=BaseClean-Linux, GuestOS=Linux,
BuildCommand='bash build.sh'). Mirrors Start-BurnInTest.ps1.

gitea/burnin-dummy/build.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# Fake build script per il repo burnin-dummy — variante Linux.
+# Simula un build reale: attende ~30s, poi scrive dist/build-info.txt.
+# Invoke-RemoteBuild impacchetta /opt/ci/build/<repo>/dist come artifacts.zip.
+# Committare questo file come build.sh nella root del repo burnin-dummy.
+set -euo pipefail
+
+DIST_DIR="$(dirname "$0")/dist"
+mkdir -p "$DIST_DIR"
+
+# Simula lavoro di build (~30 s).
+echo '[build.sh] Simulating build work (~30s)...'
+sleep 30
+
+# Scrive un artifact marker con metadati del run.
+cat > "$DIST_DIR/build-info.txt" <<EOF
+build-ok
+date=$(date '+%Y-%m-%d %H:%M:%S')
+host=$(hostname)
+pid=$$
+EOF
+
+echo "[build.sh] Artifact written to $DIST_DIR"