ui: add empty go.mod files to workspaces

Some go tools such as go list end up traversing directories that don't
have go.mod files, looking for go packages.

If you happen to have built the UI recently, that means about tens of
thousands of extra stat calls.

On macOS, this is very expensive, even with warm caches.

For example:
```

time go list -modfile=go.mod -mod=readonly -e ARGS_ELIDED  -- builtin github.com/cockroachdb/cockroach/... > /dev/null 2>&1

real    0m28.309s
user    0m23.541s
sys     0m50.926s

> for i in pkg/ui/workspaces/*/; do touch $i/go.mod; done

> time go list -modfile=go.mod -mod=readonly -e ARGS_ELIDED -- builtin github.com/cockroachdb/cockroach/... > /dev/null 2>&1

real    0m13.074s
user    0m18.282s
sys     0m19.324s
```

The go list command I used was taken from what I observed gopls
running at startup.

On Linux, the impact relatively modest.

I could avoid building the UI, but I like the UI.

Epic: none

Release note: none