Adjust agent_start default shell to use /bin/bash, with fallback to /usr/bin/env bash

[zhming0]

Description

We recently merged #3661, but we are having a second thought about using /usr/bin/env for all linux case. This PR reduce the impact of #3661 by doing a simple /bin/bash detection first before trying /usr/bin/env.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

Me and LLM

[moskyb]

LGTM!