build.sh: Remove module write-protection before rm
build.sh: Remove module write-protection before rm

If Go module support is enabled, newt's `build.sh` script fails with
messages like these:

```
+rm -rf /home/travis/gopath/bin /home/travis/gopath/pkg
rm: cannot remove ‘/home/travis/gopath/pkg/mod/golang.org/x/sys@v0.0.0-20180707002001-3c6ecd8f22c6/README.md’: Permission denied
rm: cannot remove ‘/home/travis/gopath/pkg/mod/golang.org/x/sys@v0.0.0-20180707002001-3c6ecd8f22c6/.gitattributes’: Permission denied
rm: cannot remove ‘/home/travis/gopath/pkg/mod/golang.org/x/sys@v0.0.0-20180707002001-3c6ecd8f22c6/plan9/mksysnum_plan9.sh’: Permission denied
rm: cannot remove ‘/home/travis/gopath/pkg/mod/golang.org/x/sys@v0.0.0-20180707002001-3c6ecd8f22c6/plan9/pwd_go15_plan9.go’: Permission denied
rm: cannot remove ‘/home/travis/gopath/pkg/mod/golang.org/x/sys@v0.0.0-20180707002001-3c6ecd8f22c6/plan9/syscall.go’: Permission denied
```

This is because Go write protects modules when it downloads them to
`$GOPATH/pkg`.  The fix is to relax permissions on this directory's
contents before deleting it.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt