Automatically install requirements.txt on ./build --download-dependencies

https://github.com/cirosantilli/linux-kernel-module-cheat/issues/121

build

@@ -561,6 +561,7 @@ Which components to build. Default: qemu-buildroot
                     ['python3', '-m', 'pip', 'install', '--user', LF] +
                     self.sh.add_newlines(sorted(python3_pkgs))
                 )
+            self.sh.run_cmd(['python3', '-m', 'pip', 'install', '-r', 'requirements.txt', LF])
             if ruby_pkgs:
                 # The right thing to do would be to use RVM and avoid sudo,
                 # but we felt it was too much boilerplate for now.