maint(linux): API Verification failed with unbound variable 'through' script error

```
Verify API for libkeymancore*.so (PR-16115, branch epic/kmc-test, by keyman-server):
dpkg-gensymbols: warning: debian/tmp/DEBIAN/symbols doesn't match completely debian/libkeymancore2.symbols
✔️ libkeymancore API didn't change
fatal: path 'linux/debian/libkeymancore2.symbols' exists on disk, but not in '9e2fd394fef04aa0e322be0355f2fb7efd7c0c09'
/home/runner/work/keyman/keyman/keyman/linux/scripts/verify_api.inc.sh: line 106: through: unbound variable
/home/runner/work/keyman/keyman/keyman/linux/scripts/verify_api.inc.sh: line 139: ((: > 0 : syntax error: operand expected (error token is "> 0 ")
```

https://github.com/keymanapp/keyman/actions/runs/27967442798
https://github.com/keymanapp/keyman/actions/runs/27967499255

The error appears only in the Summary tab, not in the Job log (so some output is being redirected I guess?)