Enhancement: Automatically set `GOMAXPROCS` in resource-constrained containers

When running restic in a Linux container with CPU-usage limits, restic now
automatically adjusts `GOMAXPROCS`. This helps to reduce the memory consumption
on hosts with many CPU cores.

https://github.com/restic/restic/issues/4128
https://github.com/restic/restic/pull/4485
https://github.com/restic/restic/pull/4531
