internal/lsp/cache: disable GOPACKAGESDRIVER
internal/lsp/cache: disable GOPACKAGESDRIVER

In practice, we only support the go command, and now that we rely on the
private packagesinternal APIs to set -mod and -modfile, GOPACKAGESDRIVER
is actively broken. Forcibly disable it.

Change-Id: I91f8c0d29fada2fe87ad9fdfec6ba8c5504c80cd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/268977
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools