Bugfix: Make `self-update` enabled by default only in release builds

The `self-update` command was previously included by default in all builds of
restic as opposed to only in official release builds, even if the `selfupdate`
tag was not explicitly enabled when building.

This has now been corrected, and the `self-update` command is only available
if restic was built with `-tags selfupdate` (as done for official release
builds by `build.go`).

https://github.com/restic/restic/pull/4100
