This makes gocross and its bootstrap script understand an absolute
path in go.toolchain.rev to mean "use the given toolchain directly".
Signed-off-by: David Anderson <danderson@tailscale.com>
This avoids accidentally overwriting variables from the input
environment, which might non-deterministically change the behavior
of gocross.
Signed-off-by: David Anderson <danderson@tailscale.com>