Validate: no longer require SRC_GITHUB_TOKEN to be set

[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
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

[scjohns]

LGTM, but I don't write Go code so would be great to have someone more familiar with the code give the approval

[eseliger]

did a quick glance, nothing stood out so let's go :)