chore(developer): use TeamCity repo ref to keyboards repo where we use keyboards repo in build

For example, we pull the keyboards repo for tests for kmcmplib. It would be sensible to use TeamCity's infrastructure to get this where possible, because it is cached and will save several minutes time in the build, plus remove a source of instability and traffic with large network -- 1GB+? -- pulls.

```
05:23:34  [developer/src/kmcmplib] Checking out keyboards repository at /var/lib/TeamCity/work/30ffb35c1b6c4089/keyman/developer/src/kmcmplib/tests/keyboards
05:23:34  hint: Using 'master' as the name for the initial branch. This default branch name
05:23:34  hint: is subject to change. To configure the initial branch name to use in all
05:23:34  hint: of your new repositories, which will suppress this warning, call:
05:23:34  hint:
05:23:34  hint: git config --global init.defaultBranch
05:23:34  hint:
05:23:34  hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
05:23:34  hint: 'development'. The just-created branch can be renamed via this command:
05:23:34  hint:
05:23:34  hint: git branch -m
05:23:34  Initialized empty Git repository in /var/lib/TeamCity/work/30ffb35c1b6c4089/keyman/developer/src/kmcmplib/tests/keyboards/.git/
05:23:34  [developer/src/kmcmplib] Fetching commit 965ef1941f20a3bc0cb5d405792b4984cc46cd52 in repo at /var/lib/TeamCity/work/30ffb35c1b6c4089/keyman/developer/src/kmcmplib/tests/keyboards
05:27:07  From https://github.com/keymanapp/keyboards
```

This also impacts Core test builds which depend on kmcmplib, and thus eventually all test builds. This is already supported in the build script - `KEYBOARDS_ROOT` - so we probably just need to configure that environment variable in TeamCity.