buildsys: gomod.rs has a typo in docker-go-script.sh

[Matthew James Briggs]

twoliter/tools/buildsys/src/gomod.rs

Line 51 in cdb16fa

 const GO_MOD_SCRIPT_TMPL: &str = r#".#!/bin/bash 

This line appears to be wrong. It has a dot in front of #!/bin/bash. This led us down a long detour when debugging the scripts where the real issue was something else. We saw the error .#/bin/bash does not exist, which has probably always been there, even when the script runs successfully. It's just that we would never see it unless a subsequent error happens in the script.