Fix invalid tag generation in CI

[rainest]

What this PR does / why we need it:
CI is sad; this makes it not sad.

Special notes for your reviewer:

push-public has

if: github.ref == 'refs/heads/main' || github.ref == 'refs/heads/next'

so this doesn't actually execute until the commit lands in main. This should suffice to confirm it gets what we want:

$ export GITHUB_REF="refs/heads/main"   
$ REF=`sed -re "s/^refs\/[a-z]+\///" <<< $GITHUB_REF`; echo $REF 
main

IDK much about bash's built-in pattern matching language, but brief research suggests it doesn't support the + operator or the PCRE-style classes we're using. sed definitely does, so may as well use that.