Fix invoke.sh not detecting symlinks

[max-maag]

Summary

When invoke.sh is executed using a symlink with a working directory outside of InvokeAI's root directory, it will fail with the following message:

<path to symlink>: line 23: .venv/bin/activate: No such file or directory

invoke.sh attempts to cd into the correct directory at the start of the script, but will cd into the directory of the symlink instead. This MR fixes that.

QA Instructions

• Create a symlink to invoke.sh
• cd into any directory except InvokeAI's root directory
• Execute invoke.sh using the previously created symlink to verify that the initial problem is fixed
• Execute invoke.sh directly to verify that this still works

I performed these checks on Arch Linux 6.10.3-arch1-2 (64-bit). I did not test this MR on other Linux distributions or MacOS but to my knowledge readlink is available there as well by default.

Additional Notes

A similar fix may be needed for invoke.bat. I don't have a windows machine and don't know batch well enough to be sure though.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)

[lstein]

Looks good to me! Thanks for finding and fixing this.

[psychedelicious]

Thanks!