Allow option to not build cmd/integration

In case it was built seperately (as in, on Fedora, we cannot use go
install as we do not have write permission to portions of the GOPATH)

@filbranden
This will fail if we use `set -o nounset` (which I should correct shortly.)

This would be the standard way to check whether a variable is set or unset:

```shell
if [[ "${KUBE_NO_BUILD_INTEGRATION+set}" != "set" ]]; then
  ...
```

Also, the `$(dirname $0)` needs double quotes, in fact it needs double quotes around itself and around $0 as well...

This works:

```shell
  "$(dirname "$0")"/build-go.sh cmd/integration
```

But it's really ugly... So my preference is always to set a variable at the start of the script which only needs one quote:

```shell
basedir=$(dirname "$0")
```

And then use `${basedir}` throughout the script:

```shell
source "${basedir}/util.sh"
source "${basedir}/config-go.sh"
...
if [[ "${KUBE_NO_BUILD_INTEGRATION+set}" != "set" ]]; then
  "${basedir}/build-go.sh" cmd/integration
fi
```

This script needs more love, so if you prefer we can merge your change unchanged and I can take a stab on it on a separate PR.

Cheers,
Filipe