Fixes recent scripting breeze fix to work also with zsh

[potiuk]

The BASH variable introduced in #14579 is not set when the main shell is
zsh on MacOS (which is the default) this PR changes it so that the
output of command -v bash is used instead.

Fixes #14754

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[ashb]

Curious. This script should only ever be run with bash, even when ZSH is the shell because of the #!/usr/bin/env bash shebang ling.

[potiuk]

True, in theory it should - https://tldp.org/LDP/abs/html/internalvariables.html, however I've learned not to trust MacOS environment and be a little defensive (I am not 100% sure what was the scenario that led to BASH env missing - but taking into account the large variations of Bash's there for MacOS (the built-in 3.2, any version that can be installed by brew etc.) - command -v bash is all but guaranteed to work.

But yeah - it might be an interesting question how it got there.