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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def build_src(ctx, dest=None):
 """ build source archive """

    if dest:
        if not dest.startswith('/'):
            # Relative
            dest = os.path.join(os.getcwd(), dest)

        os.chdir(PROJECT_DIR)
        ctx.run('python setup.py sdist --dist-dir {0}'.format(dest))
    else:
        os.chdir(PROJECT_DIR)
        ctx.run('python setup.py sdist')