test_runner: set AWS credentials for endpoints (#4887)

## Problem

If AWS credentials are not set locally (via
AWS_ACCESS_KEY_ID/AWS_SECRET_ACCESS_KEY env vars)
`test_remote_library[release-pg15-mock_s3]` test fails with the
following error:

```
ERROR could not start the compute node: Failed to download a remote file: Failed to download S3 object: failed to construct request
```

## Summary of changes
- set AWS credentials for endpoints programmatically

test_runner/fixtures/neon_fixtures.py

@@ -1595,7 +1595,11 @@ def endpoint_start(
         if endpoint_id is not None:
             args.append(endpoint_id)
 
-        res = self.raw_cli(args)
+        s3_env_vars = None
+        if self.env.remote_storage is not None and isinstance(self.env.remote_storage, S3Storage):
+            s3_env_vars = self.env.remote_storage.access_env_vars()
+
+        res = self.raw_cli(args, extra_env_vars=s3_env_vars)
         res.check_returncode()
         return res
 

test_runner/regress/test_compatibility.py

@@ -337,6 +337,7 @@ def check_neon_works(
     config.pg_version = pg_version
     config.initial_tenant = snapshot_config["default_tenant_id"]
     config.pg_distrib_dir = pg_distrib_dir
+    config.remote_storage = None
 
     # Use the "target" binaries to launch the storage nodes
     config_target = config