knotserver: fix air ldflags path for version injection
.air/knot.toml targets tangled.org/core/knotserver.version, but the
actual version variable is at tangled.org/core/knotserver/xrpc.version
(knotserver/xrpc/version.go:12). Dev builds via air therefore never
set the version variable, falling through to the debug.ReadBuildInfo()
fallback in version.go.
Signed-off-by: Anish Lakhwara <anish+git@lakhwara.com>