devel/tinygo: Add explicit Python build dependency (D29353)

This unbreaks the build when using devel/samurai.

It previously got it through ninja which has a run dependency on
Python.

• Could NOT find PythonInterp (missing: PYTHON_EXECUTABLE)

CMake Error at CMakeLists.txt:652 (message):

Unable to find Python interpreter, required for builds and testing.

Please install Python or specify the PYTHON_EXECUTABLE CMake variable.

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/tinygo-0.14.1_1.log

PR: 254678