Added ZSH support for micromamba

[Makkareal]

Why:
When using Zsh source source .ci/micromamba/init.sh did not work, because it did not find the location of the script.

What:
Added a branch if the shell is Zsh

Addresses:
None

[furtib]

Hello, welcome to the project!

There are multiple bash script files using this or similar methods for obtaining the location of the script (e.g., .ci/micromamba/run_test.sh, .ci/micromamba/uninstall.sh); those should be updated too.
If possible, I suggest avoiding the ZSH-specific execution path and using something that works universally. (The solution I used in run_test.sh seems to work in ZSH too, please check that! -- does not work with sourcing)

I'm slightly concerned about the impact of ZSH support inside basel.

Since we are not explicitly supporting ZSH, I relabeled this as an enhancement, not a bug.

[furtib]

This line does not work in ZSH; see the following example script:

furtib@emypc ~ % cat test.sh 
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    echo "ERROR: This script must be 'sourced':"
    echo "       source ${0}"
    exit 1
fi


furtib@mypc ~ % ./test.sh
./test.sh: 1: Bad substitution

[Makkareal]

Added shebang, so the script always runs in bash when not sourced.

Unfortunately, I could not find a universal way of getting the correct path. If the support of ZSH is a concern, then perhaps it would be better to simply check if the script is sourced from bash and exit with an instruction if it is not.

I will update the other scripts with whichever solution is preferred.