go-md2man: disable module-mode (#12985)

* go-md2man: disable module-mode

* Update var/spack/repos/builtin/packages/go-md2man/package.py

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/go-md2man/package.py

@@ -37,6 +37,7 @@ def install(self, spec, prefix):
 
         with working_dir('src'):
             env['GOPATH'] = self.stage.source_path
+            env['GO111MODULE'] = 'off'
             go = which('go')
             go('build', '-v', join_path(
                'github.com', 'cpuguy83', 'go-md2man'))