feat(magrathea): add flag passthru for arguments to nix-build r/3994

in some cases, users might want to pass through flags for
nix-build (such as `-j`).

magrathea now accepts these as arguments to `mg build`, as long as
they are separated by `--`.

the arguments passed to `mg build` are parsed into a proper record,
which enables us to show users very clear error messages in case they
forget to use the `--` separator and keeping us future-compatible with
more potential arguments to magrathea itself.

Change-Id: I81f5d9db52779a5cc3b8bbdd975316274fffe5fc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5507
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>
Reviewed-by: asmundo <asmundo@gmail.com>

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