fix: fix build.linux.desktop.entry

[h-banii]

This fixes the build.linux.desktop.entry in the package.json

According to electron-builder's documentation, the entry should be an object describing the desktop entry
https://www.electron.build/app-builder-lib.interface.linuxdesktopfile#entry

Since commit 8de2735, the entry is being set to a string, so the .desktop file generated has bogus key-values.

before this PR

[Desktop Entry]
0=c
1=o
2=m
3=.
4=g
5=i
6=t
7=h
8=u
9=b
10=.
11=t
12=h
13=_
14=c
15=h
16=.
17=y
18=o
19=u
20=t
21=u
22=b
23=e
24=_
25=m
26=u
27=s
28=i
29=c
Name=YouTube Music
Exec=AppRun --no-sandbox %U
Terminal=false
Type=Application
Icon=youtube-music
StartupWMClass=YouTube Music
X-AppImage-Version=3.7.1
Comment=YouTube Music Desktop App bundled with custom plugins (and built-in ad blocker / downloader)
Categories=AudioVideo;

after this PR

[Desktop Entry]
Name=YouTube Music
Exec=AppRun --no-sandbox %U
Terminal=false
Type=Application
Icon=youtube-music
StartupWMClass=com.github.th_ch.youtube_music
X-AppImage-Version=3.7.1
Comment=YouTube Music Desktop App bundled with custom plugins (and built-in ad blocker / downloader)
Categories=AudioVideo;

(.desktop files above were extracted from the AppImages generated)

[JellyBrick]

Thanks!