Bugfix: Return error if environment variables contain invalid values

If the value of one of the environment variables `RESTIC_COMPRESSION`, `RESTIC_PACK_SIZE`
or `RESTIC_READ_CONCURRENCY` could not be parsed, then restic ignored their value.
Now, the restic commands fail with an error, unless they were overwritten with their
corresponding command-line option.

https://github.com/restic/restic/issues/4759
https://github.com/restic/restic/pull/5592
https://github.com/restic/restic/pull/5700