Standardise bash shebang

[alcaeus]

Related to DRIVERS-2852

The GitHub Action does not explicitly use a shell when calling run-orchestration.sh, but instead relies on the shebang to choose the appropriate shell. This causes warnings when running the action:

/home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/run-orchestration.sh: 23: /home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/handle-paths.sh: [[: not found
/home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/run-orchestration.sh: 40: /home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/handle-paths.sh: [[: not found
/home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/run-orchestration.sh: 59: /home/runner/work/drivers-evergreen-tools/drivers-evergreen-tools/.//.evergreen/handle-paths.sh: [[: not found

This PR changes all shebangs to use /usr/bin/env bash, so that any invocations of scripts that do not explicitly use sh (as is the case in some evergreen configs) use bash. This removes warnings above.

Note that any drivers that have not yet completed DRIVERS-2852 (currently C, C++, Node, Ruby, Rust) might see changes in behaviour on Evergreen if they do not use an explicit shell when invoking run-orchestration.sh or any other script changed here. I don't think there are any negative side-effects to this, but wanted to point it out.

Testing

To test this PR, change the uses key in the GitHub Action Workflow using this action to the following, then run the workflow:

uses: alcaeus/drivers-evergreen-tools@use-bash-shebang

A green build with no warnings indicates that this PR does what it's supposed to do.

[blink1073]

LGTM!