`sync-template` should run against the referenced revision

Currently, `sync-template` runs `nix flake init --template "github:sellout/flaky-environments#…`, which means that doing

```bash
nix run .#sync-template
```
or 
```bash
nix run github:sellout/flaky-environments/some-experimental-branch
```
or
```bash
nix run github:someone-else/flaky-environments
```
makes no difference.

But it _should_ run against the template in the revision we run against.
