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>

.builds/linux.yml

@@ -46,7 +46,8 @@ tasks:
      cd gio/cmd
      go test ./...
      go test -race ./...
-     go test -modfile=go.local.mod ./...
+     cd gogio # since we need -modfile to point at the parent directory
+     GOFLAGS=-modfile=../go.local.mod go test
  - test_example: |
      cd gio/example
      go test -race ./...