ci : set GITHUB_ACTION env var for server tests (#12162)

This commit tries to address/improve an issue with the server tests
which are failing with a timeout. Looking at the logs it seems like
they are timing out after 12 seconds:
```
FAILED unit/test_chat_completion.py::test_completion_with_json_schema[False-json_schema0-6-"42"] - TimeoutError: Server did not start within 12 seconds
```

This is somewhat strange as in utils.py we have the following values:
```python
DEFAULT_HTTP_TIMEOUT = 12

if "LLAMA_SANITIZE" in os.environ or "GITHUB_ACTION" in os.environ:
    DEFAULT_HTTP_TIMEOUT = 30

    def start(self, timeout_seconds: int | None = DEFAULT_HTTP_TIMEOUT) -> None:
```
It should be the case that a test running in a github action should have
a timeout of 30 seconds. However, it seems like this is not the case.
Inspecting the logs from the CI job we can see the following environment
variables:
```console
Run cd examples/server/tests
2 cd examples/server/tests
3 ./tests.sh
4 shell: /usr/bin/bash -e {0}
5 env:
6 LLAMA_LOG_COLORS: 1
7 LLAMA_LOG_PREFIX: 1
8 LLAMA_LOG_TIMESTAMPS: 1
9 LLAMA_LOG_VERBOSITY: 10
10 pythonLocation: /opt/hostedtoolcache/Python/3.11.11/x64
```

This probably does not address the underlying issue that the servers
that are providing the models to be downloaded occasionally take a
longer time to response but might improve these situations in some
cases.

.github/workflows/server.yml

@@ -161,6 +161,8 @@ jobs:
       - name: Tests
         id: server_integration_tests
         if: ${{ matrix.sanitizer == '' }}
+        env:
+          GITHUB_ACTIONS: "true"
         run: |
           cd examples/server/tests
           ./tests.sh