Change to `WORKDIR` before running commands in cloud project experiments. (#10918)

Mitigates the known issue where we don't automatically change to the
`WORKDIR` defined in `Dockerfile` when running cloud experiments.

Question:
Would it be preferred if I introduce a flag for this?
(e.g., `--use_workdir` or `--workdir=/src/<project>`)
While this gives more flexibility, I feel `cd` to `WORKDIR` should
always be preferred if we want the cloud experiments to behave the same
as local ones.

infra/build/functions/project_experiment.py

@@ -72,11 +72,12 @@ def run_experiment(project_name, command, output_path, experiment_name):
           ]
       },
       {
-          'name': project.image,
+          'name':
+              project.image,
           'args': [
               'bash',
               '-c',
-              command,
+              f'(cd "/src"; cd {project.workdir}; {command})',
           ]
       },
       {