Convert zstd shelling to klauspost/compress

[joestringer]

PR #184 will add a dependency on klauspost/compress for zstd handling. Since we'll have this dependency already, we should switch the following shell-out code to use the same Go library:

little-vm-helper/pkg/images/pull.go

Line 181 in 31f007a

cmd := exec.CommandContext(ctx, "zstd", "-d", "-", "-o", tmpPath, "-f")

Link: https://github.com/klauspost/compress