Breaking change in Palace makes CI fail

[David-GERARD]

Describe the bug

This commit awslabs/palace@cec1c1d removed the sigularity folder from the Palace repo, causing the CI workflows to fail.

To Reproduce

 - name: Download Palace singularity definition
        run: wget https://raw.githubusercontent.com/awslabs/palace/main/singularity/singularity.def

Suggested fix

Either:

• Pin a previous version of the repository
• Update to new workflow