DRY up the lengthy .tar file name

The name of the `.tar` file is repeated multiple times, verbatim. This
not only violates the Don't Repeat Yourself (DRY) principle, it also
makes it easier to miss an instance when the file name changes (as it
did with lycheeverse/lychee#1464).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

action.yml

@@ -48,9 +48,10 @@ runs:
       run: |
         # Cleanup artifacts from previous run in case it crashed
         rm -rf "lychee-${{ inputs.LYCHEEVERSION }}-x86_64-unknown-linux-gnu.tar.gz" lychee
-        curl -sfLO "https://github.com/lycheeverse/lychee/releases/download/${{ inputs.LYCHEEVERSION }}/lychee-${{ inputs.LYCHEEVERSION }}-x86_64-unknown-linux-gnu.tar.gz"
-        tar -xvzf "lychee-${{ inputs.LYCHEEVERSION }}-x86_64-unknown-linux-gnu.tar.gz"
-        rm "lychee-${{ inputs.LYCHEEVERSION }}-x86_64-unknown-linux-gnu.tar.gz"
+        filename='lychee-${{ inputs.LYCHEEVERSION }}-x86_64-unknown-linux-gnu.tar.gz'
+        curl -sfLO "https://github.com/lycheeverse/lychee/releases/download/${{ inputs.LYCHEEVERSION }}/$filename"
+        tar -xvzf "$filename"
+        rm "$filename"
         install -t "$HOME/.local/bin" -D lychee
         rm lychee
         echo "$HOME/.local/bin" >> "$GITHUB_PATH"