Fix depot_tools initialization in linux build script

Explicitly run `ensure_bootstrap` in `linux/build_linux.sh` to ensure `depot_tools` is properly initialized before running `gclient`. This fixes the `python3_bin_reldir.txt not found` error during `gn gen`.

Author: google-labs-jules[bot]

linux/build_linux.sh

@@ -32,6 +32,11 @@ export PATH="$PATH:$(pwd)/depot_tools"
 
 output "Initializing depot_tools..."
 
+# Explicitly ensure bootstrap
+if [ -f "depot_tools/ensure_bootstrap" ]; then
+    depot_tools/ensure_bootstrap
+fi
+
 gclient || (output "gclient initialization failed!" && exit 1)
 
 output "All tools are installed and initialized"