cmd/go: do not attempt to install cmd/addr2line in TestScript/mod_outside
Tests must not write to GOROOT: it might not writable (for example, if
it is owned by root and the user is non-root), and in general we can't
assume that the configuration in which the test is run matches the
configuration with which the installed tools were built.
In this specific case, CL 454836 (for #57007) installs 'cmd' with
CGO_ENABLED=0, but most builders still run the tests with CGO_ENABLED
unset.