Update script to retain CNAME file

[jcleigh]

The existing entrypoint.sh script removes historical builds, which also removes the CNAME file created by setting a custom domain name in GitHub Pages settings. Added grep -v 'CNAME' to remove that filename (if it exists) from the file list pipe.

Local shell testing verifies it works. I prepared an empty directory with:

touch 1
touch 2
touch 5
touch 10
touch CNAME

Here is the result of the original code:

> ls
1  10  2  5  CNAME
> ls | sort -n
CNAME
1
2
5
10
> ls | sort -n | head -n -2
CNAME
1
2
> ls | sort -n | head -n -2 | xargs rm -rv
removed 'CNAME'
removed '1'
removed '2'
> ls
10  5

Here is the updated code with grep -v 'CNAME' in the pipe:

> ls
1  10  2  5  CNAME
> ls | sort -n | grep -v 'CNAME' | head -n -2 | xargs rm -rv
removed '1'
removed '2'
> ls
10  5  CNAME

Here is the updated code but CNAME file does not exist, showing that this change is non-breaking:

> ls
1  10  2  5
> ls | sort -n | grep -v 'CNAME' | head -n -2 | xargs rm -rv
removed '1'
removed '2'
> ls
10  5

I have also run Actions against my company's private repo pointed at my change branch and the CNAME file is no longer being deleted.

[jcleigh]

@simple-elf Are you able to review or provide feedback on this? Thanks!