Fix "Desktop file didn't specify Exec field" error on Ubuntu 24.04

Backport of zotero/zotero#4163

linux/zotero.desktop

 [Desktop Entry]
 Name=Zotero
-Exec=bash -c "$(dirname $(realpath $(echo %k | sed -e 's/^file:\/\///')))/zotero -url %U"
+Exec=bash -c "$(dirname $(realpath $(echo %k | sed -e 's/^file:\\/\\///')))/zotero -url %U"
 Icon=zotero.ico
 Type=Application
 Terminal=false