Reinstate selfupdate tag to make builds without self-update work

Revert what seems to be a typo introduced as part of the fix for #2041
in 2018 7d0f2eaf24.

`xbuild` does not look like a go build/tag keyword to me, I failed to
find documentation for it and using `go install -tags '!selfupdate' ...`
has no effect, i.e. self-update code is still compiled.

`+build` however works;  updating the OpenBSD port/binary package
security/restic to apply this PR works as expected:

```
	$ restic help | grep self
	$ restic self-update
	unknown command "self-update" for "restic"
```

(Using `go:build` now as per restic's style and gofmt.)

Previously, using `restic-0.14.0p1` on OpenBSD/amd64 7.2-current would
check for a newer version and probably attempt replacing the system wide
root-owned executable (on a read-only filesystem) as unprivileged user:

```
	$ restic version
	restic 0.14.0 compiled with go1.19.2 on openbsd/amd64
	$ restic help | grep self
	  self-update   Update the restic binary
	$ restic self-update
	writing restic to /usr/local/bin/restic
	find latest release of restic at GitHub
	restic is up to date
```

(It never tried to actually write besaid path;  doing so would fail, so
the current message can be considered misleading.)

cmd/restic/cmd_self_update.go

@@ -1,4 +1,4 @@
-// xbuild selfupdate
+//go:build selfupdate
 
 package main