Possible risk of temporary file collision

[JensRantil]

I'm reading

gleam/filesystem/vfs_s3.go

Line 83 in 766b221

filename := fmt.Sprintf("%s/s3_%d", os.TempDir(), rand.Uint32())

and

gleam/filesystem/vfs_gs.go

Line 108 in 53a6956

filename := fmt.Sprintf("%s/gs_%d", os.TempDir(), rand.Uint32())

and both has a slight risk of file name collisions. I propose migrating to use https://golang.org/pkg/io/ioutil/#TempFile which makes that scenario impossible.

[chrislusf]

Send a PR?