Bugfix: Error out when environment variables hold invalid values

If the environment variables `RESTIC_COMPRESSION`, `RESTIC_PACK_SIZE`, or
`RESTIC_READ_CONCURRENCY` could not be parsed, restic used to ignore them.
Restic now fails with an error unless the same setting is overridden on the
command line.

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