Commit a8b927a562 Added new procedure go-module-available-versions use
'string-append' to GOPROXY, but 'string->symbol' let GOPROXY is a symbol (it
must be a string), which would lead to wrong-type-arg errors in the
'string-append' procedure.
* guix/scripts/import/go.scm (%options)[goproxy]: Remove call to
'string->symbol'.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>