Validate: no longer require SRC_GITHUB_TOKEN to be set

Created by: jdpleiness

Previously src validate install required the env var SRC_GITHUB_TOKEN to be set, even if no external services were being added. This also blocked validating queries with repositories that already existed on a Sourcegraph deployment.

This fix allows src validate install to be run without requiring the SRC_GITHUB_TOKEN env var to be set, as well as validating queries with repositories that already exist on a deployment.

Fixes #956 (closed) Fixes https://github.com/sourcegraph/sourcegraph/issues/47576

Test plan

Locally tested changes using latest Sourcegraph Helm install.

ran go test ./... - all tests passing ran staticcheck ./... - no issues found in new code