📚 Fix manual CI to handle branches and tags

Change the reference computation to use a `sed` call instead of bash
variable trimming so that it can handle tags and remove the slashes from
tag paths as well as branch refs.

Also remove potential slashes from either tags or branch names.

.github/workflows/manual.yaml

@@ -18,7 +18,7 @@ jobs:
         shell: bash
         run: |
           echo "sha_short=$(git rev-parse --short "$GITHUB_SHA")" >> "$GITHUB_ENV"
-          echo "branch=$(echo ${GITHUB_REF#refs/heads/})" >> "$GITHUB_ENV"
+          echo "branch=$(echo $GITHUB_REF | sed 's/refs\/\(tags\|heads\)\///,s/\//_/g')" >> "$GITHUB_ENV"
 
       # Install mdbook
       - name: Install `mdbook`