Starting with zstd-1.3.8 we support the `ZSTD_CLEVEL` environment variable. We've started with a small scope for the variable, because we don't want users to unexpectedly remove the source fie, for instance.

If you want to pass extra options, you can pipe the output to zstd, which is exactly what tar is doing internally.

Last time I checked zstd does not support getting options from an environment variable (unlike .e.g. gzip).

With a new enough version of tar you can pass a command with arguments to '-I' (e.g. `tar -I 'zstd -19' ...`).

The alternative is piping the output from tar through zstd yourself.