x/tools/gopls: go mod tidy diagnostic fails with tags in buildFlags

[ian-mi]

What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

Loaded package in gopls

What did you expect to see?

No errors

What did you see instead?

2020/10/05 20:41:16 warning: diagnose go.mod: err: exit status 2: stderr: flag provided but not defined: -tags
usage: go mod tidy [-v]
Run 'go help mod tidy' for details.

[ian-mi]

I also see a similar error running go version when opening a workspace:

LSP :: Error loading workspace folders (expected 1, got 1)
failed to load view for file:///...: err: exit status 2: stderr: flag provided but not defined: -tags
usage: go version [-m] [-v] [file ...]
Run 'go help version' for details.

[gopherbot]

Change https://golang.org/cl/260000 mentions this issue: internal/lsp: don't pass build flags to go mod commands