Better handling for git not being installed during service push

**Intended outcome:**

Attempting to push schema using the CLI from a docker image that does not have git CLI installed.

I am providing a variant name (a.k.a 'tag'), so the git commit hash should not be necessary. I should see a successful push without any errors.

The process does continues and completes successfully, but I need to supply this tool to colleagues and an error in the logs is going to cause issues.

**Actual outcome:**

An error is output to the console.

```
Loading Apollo Project [started]
Loading Apollo Project [completed]
Uploading service to Apollo Graph Manager [started]
Unable to retrieve remote url, failed with:
Error: [git-rev-sync] failed to execute command: null/Error: spawnSync git ENOENT
Fetching info from federated service
Uploading service to Apollo Graph Manager [completed]
A new service called 'test' for the 'my-graph@dev' graph was created
The gateway for the 'my-graph@dev' graph was updated with a new schema, composed from the updated 'orders' service
╔════════╤══════════════════════╤═══════════════╗
║ id │ graph │ tag ║
╟────────┼──────────────────────┼───────────────╢
║ 72d97a │ my-graph │ dev ║
╚════════╧══════════════════════╧═══════════════╝
```

**How to reproduce the issue:**

Execute the service:push command in an environment where git is not installed. Use the --variant argument.

**Versions**

apollo@2.28.0

Start at the service:push command and reproduce the upload in an environment without git, using --variant. Trace the handling of the git-rev-sync ENOENT message; done means a variant-based push succeeds without emitting an error when git is unavailable.

Written by the indexing model from the issue text.