x/tools/gopls: enable -mod=readonly in workspace module mode

[findleyr]

When using multi-project gopls workspaces, invoking the go command with -mod=readonly fails because the go command wants to modify the synthetic versions we use in the workspace module's go.mod file.

We should tidy as part of constructing the workspace module.

CC @stamblerre @heschik

[gopherbot]

Change https://golang.org/cl/287032 mentions this issue: internal/lsp: enable -mod=readonly in workspace module mode