x/tools/gopls: allowModfileModifications when using tempModFile or go.work

[findleyr]

govim uses an internal gopls setting: tempModFile, along with allowModfileModifications. This appears to be recently broken, perhaps by https://go.dev/cl/382241.

Reported in slack, from their CI:

&protocol.LogMessageParams{Type:1, Message:"2022/02/07 17:40:55 imports fixes: AllImportsFixes: -modfile is unsupported for this Go version\n\tfile=\"/tmp/go-test-script3721705982/script-initial_open_go/main.go\"\n"}

To begin with, this error message is incorrect, as -modfile is supported.

Needs investigation, and should block gopls@v0.8.0

CC @myitcv @leitzler

[gopherbot]

Change https://go.dev/cl/388436 mentions this issue: gopls/integration/govim: build gopls using go1.18rc1

[findleyr]

I have confirmed that our gopls+govim CI is now passing (after updating our build to use RC1).

I think allowModfileModifications may still be buggy, but that shouldn't block the gopls@v0.8.0 release.