fix: remove breaking spaces exported desktop files

[simon-wg]

I noticed some issues when exporting vscode to my host.

Due to not having any of the rootful/enter_flags/sudo_prefix options set, the Exec command of the exported desktop file had too many spaces, not following standards, and causing bemenu to be unable to launch them.
The same issue was present for the extra_flags.

The solution was to use shell parameter expansion instead of just listing the parameters.

Examples before the fix:

• Exporting an app.
  Exec=/usr/bin/distrobox-enter -n devel -- /usr/share/code/code %F
• Generating desktop entry
  Exec=/usr/bin/distrobox enter devel

After fix:

• Exporting an app.
  Exec=/usr/bin/distrobox-enter -n devel -- /usr/share/code/code %F
• Generating desktop entry
  Exec=/usr/bin/distrobox enter devel

[dottorblaster]

Did you try replicating the behavior on the next branch? We gladly accept fixes there

[simon-wg]

Oh sorry, I didn't read the part about a complete rewrite in go.
I'm able to replicate the behavior on next as well, at least for exporting, so I'll rebase my changes to the script there!

Edit: Was able to replicate for generate-entry as well, so have ported that fix to go templates.

[dottorblaster]

Thank you so much!