buildGoModule: remove cached lookup results and tiles

Since GO 1.13, the go command caches the lookup results and tiles in
$GOPATH[1], hence making the module directory non-deterministic.

Use the `-f` flag when removing /sumdb, for compatibility with Go 1.12
because in that version does not exists that directory.

[1] https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md#command-client