go/packages: remove -mod, -modfile flags from build flags for go version
go/packages: remove -mod, -modfile flags from build flags for go version

Use provided build flags may include -mod or -modfile, and these should
be removed before any go command calls that don't accept them.

Fixes golang/go#43418

Change-Id: Ie96626bced5093c67fc1890533f1f8cc03d42c80
Reviewed-on: https://go-review.googlesource.com/c/tools/+/280694
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

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