shell.nix: use tailscale-go for compilation

This change builds a derivation for tailscale-go and makes it available in the
users development environment. This is consistent with the shell.nix in corp/.

Once go1.18 is in a stable Nixpkgs release we can avoid relying on derivations
from nixpkgs head. For now, this works well, and the fetched derivations are
cached in the Nix store according to the usual rules.

Fixes #4231

Signed-off-by: Tom DNetto <tom@tailscale.com>
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