Dockerfile: remove long-obsolete -tags=xversion
That build tag hasn't been used since
(June 2nd, 2021, for 1.10.0)
@@ -50,7 +50,7 @@ ARG VERSION_GIT_HASH=""
ENV VERSION_GIT_HASH=$VERSION_GIT_HASH
ARG TARGETARCH
-RUN GOARCH=$TARGETARCH go install -tags=xversion -ldflags="\
+RUN GOARCH=$TARGETARCH go install -ldflags="\
-X tailscale.com/version.Long=$VERSION_LONG \
-X tailscale.com/version.Short=$VERSION_SHORT \
-X tailscale.com/version.GitCommit=$VERSION_GIT_HASH" \
