cmd: bump gioui.org dependency, use -modfile properly on CI

A simple 'go test -modfile=<path>' won't work properly for the
end-to-end tests, since they run 'go build' under the hood as well.

To properly propagate the flag, we need to use GOFLAGS. Since -modfile
is always relative to the current directory, we can't use it to test
many packages at once, nor can we use it via 'go test ./gogio'.

While at it, document this distinction in go.local.mod to prevent others
getting confused like I did.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/go.local.mod

@@ -1,3 +1,9 @@
+// Keep in sync with go.mod; the only difference should be the gioui.org
+// replace. To use it:
+//
+//     cd gogio
+//     GOFLAGS=-modfile=../go.local.mod go test
+
 module gioui.org/cmd
 
 go 1.13