Use the official Go binaries for bootstrapping

We've seen that since Go is completely reproducible, we can download the official binaries, at least for bootstrapping purposes, rather than starting from the last compiler written in C and adding more and more versions.

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Original description:

Using Debian's go to build go-bootstrap avoids building go1.4 using the C compiler. Also, since go1.4 does not support the arm64 architecture, it is the only way to achieve a fully native build on arm64.