cmd/go: use local state object in work.runBuild and work.runInstall

cmd/go: use local state object in work.runBuild and work.runInstall

This commit modifies `runBuild` and `runInstall` to construct a new
modload.State object using the new constructor instead of the current
global `modload.LoaderState` variable.

This commit is part of the overall effort to eliminate global
modloader state.

[git-generate]
cd src/cmd/go/internal/work
rf '
  add build.go:/func runBuild\(/-0 var moduleLoaderState *modload.State
  ex {
    import "cmd/go/internal/modload";
    modload.LoaderState -> moduleLoaderState
  }
  add runBuild://+0 moduleLoaderState := modload.NewState()
  add runInstall://+0 moduleLoaderState := modload.NewState()
  rm build.go:/var moduleLoaderState \*modload.State/
'

Change-Id: I1137e0b898a5bda8697dce8713f96f238ae8b76c
Reviewed-on: https://go-review.googlesource.com/c/go/+/711135
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>