Call bash only when necessary on macOS

[cdalvaro]

What does this PR do?

This PR fixes a bug introduced on PR #54136 (and possibly on PR #54079 too) that calls /bin/bash when the user's default shell is not bash.

Also, users that have set a different version of bash as their default shell, they might be calling the wrong shell.

For example, for people with zsh as default shell the current behaviour is not right, neither for people with /usr/local/bin/bash as their main shell since the current code is invoking /bin/bash instead the right one.

New Behavior

The new behaviour introduced in this PR invokes the right bash shell only when the users default shell is bash.

This is done in this way to load the right environment as it was intended in PR #54136.

Tests written?

✅

Commits signed with GPG?

✅

[cdalvaro]

Following @waynew advice from PR #54216

Hey @cdalvaro - as you may have heard, we're changing to a single branch release strategy. What does that mean for this PR?

Well, you've got tests so 👍

We will be working to migrate all PRs from non-master branches to master. If you'd like to get your PR in sooner, please rebase it against master, force push to your branch, and edit this PR to be against master. If you need any help with any of those steps, please let us know!

I have modified this PR too in order to fulfil the new release strategy.

[cdalvaro]

I would like to know if this PR will be finally merged

[cdalvaro]

Please @xeacott, could you revise this PR, it is open for nearly two months.

Thank you in advance.

[weswhet]

Looks good to me!

[cdalvaro]

Please @xeacott, could you revise this PR, it solves an issue introduced in #54136.

By accepting this pull request, #54079 could be closed since it was duplicated in #54136 in order to rebased commits into 2019.2.1 instead of 2019.2

I mention @weswhet too, because I don't know if @xeacott is actively working on this project since I haven't heard from him for a long time.

Thank you all in advance!!