client: fix deadlock when connect to buildkitd fails

Before this change, if BUILDKIT_HOST was set to an invalid value that
resulted in the connection of the grpc client to the server to fail,
then deadlock occured waiting for the `eventsCh` to be closed.

This happened because the call to the buildkit client's Build method
returned an error before the provided callback was executed, which is
where the `eventsCh` gets closed.

This change places the creation of `eventsCh` inside the Build callback,
which avoids the increment of the WaitGroup and thus the deadlock in
this error path.

Signed-off-by: Erik Sipsma <erik@sipsma.dev>

tests/plan.bats

@@ -248,3 +248,13 @@ setup() {
    assert_failure
    assert_output --partial "no match for platform in manifest"
 }
+
+@test "plan/do: invalid BUILDKIT_HOST results in error" {
+   cd "$TESTDIR"
+
+   # ip address is in a reserved range that should be unroutable
+   export BUILDKIT_HOST=tcp://192.0.2.1:1234
+   run timeout 30 "$DAGGER" "do" -p ./plan/do/actions.cue test
+   assert_failure
+   assert_output --partial "Unavailable: connection error"
+}