Element nightly: element-desktop-nightly.desktop contains invalid value of StartupWMClass thus element is not docked properly

[doubleloop]

Describe the bug

$ cat /usr/share/applications/element-desktop-nightly.desktop
[Desktop Entry]
Name=Element-Nightly
Exec=/opt/Element-Nightly/element-desktop-nightly %U
Terminal=false
Type=Application
Icon=element-desktop-nightly
StartupWMClass=element
Comment=A feature-rich client for Matrix.org
MimeType=x-scheme-handler/element;
Categories=Network;InstantMessaging;Chat;

For nightly build, StartupWMClass should be element-nightly instead of element.

This for example causes duplicated element icon in gnome dock panel:

ref: https://askubuntu.com/questions/367396/what-does-the-startupwmclass-field-of-a-desktop-file-represent/367851#367851

I believe the value is taken from package.json
https://github.com/vector-im/element-desktop/blob/f54249146747873df9f0da3a9833b7f35aa1b5e4/package.json#L135

Note: also Description could reflect the value specified in:
https://github.com/vector-im/element-desktop/blob/f54249146747873df9f0da3a9833b7f35aa1b5e4/element.io/nightly/control.template#L11

[t3chguy]

Duplicate of element-hq/element-web#13780