Fix CI env export: glob matched GNU ELPA's .signed marker too

The dash install succeeded; the step failed writing GITHUB_ENV because
ls -d .elpa/dash-* matched both the package directory and the adjacent
dash-N.N.N.signed marker, producing a second non-KEY=value line
("Invalid format"). Restrict the glob to directories. Also bump
actions/checkout to v6 to clear the Node 20 deprecation warning.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         emacs_version: ['28.1', '29.4', '30.1']
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - uses: purcell/setup-emacs@master
         with:
@@ -29,7 +29,9 @@ jobs:
             (package-initialize) \
             (package-refresh-contents) \
             (package-install 'dash))"
-          echo "LOAD_EXTRA=-L $(ls -d "$PWD"/.elpa/dash-*)" >> "$GITHUB_ENV"
+          # Trailing slash: match only the package directory, not the
+          # adjacent dash-N.N.N.signed marker GNU ELPA leaves behind.
+          echo "LOAD_EXTRA=-L $(ls -d "$PWD"/.elpa/dash-*/ | head -1)" >> "$GITHUB_ENV"
 
       - name: Byte-compile (warnings are errors)
         run: make compile-all WERROR=t LOAD_EXTRA="$LOAD_EXTRA"