Add script to force build LUCI builders for a specified commit (#15938)

tools/luci/force_luci_build.sh

+#!/bin/bash
+
+if [[ -z "$1" ]]; then
+    echo "Usage: $(basename $0) <engine_commit_hash>"
+    exit 1
+fi
+
+ENGINE_COMMIT=$1
+BUILDERS=$(curl 'https://ci.chromium.org/p/flutter/g/engine/builders' 2>/dev/null|sed -En 's:.*aria-label="builder buildbucket/luci\.flutter\.prod/([^/]+)".*:\1:p'|sort|uniq)
+
+IFS=$'\n'
+for BUILDER in $BUILDERS; do
+    echo "Building $BUILDER..."
+    bb add \
+       -commit "https://chromium.googlesource.com/external/github.com/flutter/engine/+/$ENGINE_COMMIT" \
+       "flutter/prod/$BUILDER"
+    sleep 1
+done