x/build: ensure that the coordinator attempts to connect with the buildlet for 5 minutes

[cagedmantis]

When a VM instance is created, the coordinator should attempt to connect to the buildlet for either 5 minutes or the timeout of the context passed to the buildletClient function.

After change golang/build@372ecfe, the buildlet client will only attempt to connect for 5 seconds before failing to create a buildlet client.

/cc @dmitshur @toothrot @andybons

[gopherbot]

Change https://golang.org/cl/232997 mentions this issue: buildlet: ensure buildlet client creation timeout is correct