Dockerfile: remove long-obsolete -tags=xversion

That build tag hasn't been used since 5088af68cf
(June 2nd, 2021, for 1.10.0)

Change-Id: Ib6093a975505339872a0dc663fff9fc09c13bbc0

Dockerfile

@@ -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" \