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.)
94f6e7d4a6