Fix npm run build-x86 command in nsventforwarder

[MarkusPettersson98]

This PR fixes an issue introduced in #7211 where a an extra dash (-) snuck into the build-x86 npm command in the nsventforwarder workspace. This messed up the variable substitution, which tripped up cp because we provided it with a nonsensical path.

• Bad build-x86: cp ${CARGO_TARGET_DIR:--../../../target}
• Good build-x86: cp ${CARGO_TARGET_DIR:-../../../target}

The consequence of this is that we can not package the x86-version of the macOS app, which subsequently disallows us from producing universal installers.

I've double-checked that this PR actually fixes the problem, and here is the proof:

$ ./build.sh --optimize --universal
..
  • building        target=pkg arch=universal file=/Users/markus/projects/mullvadvpn-app/dist/MullvadVPN-2024.7-dev-f01e57.pkg
[14:29:20] Finished 'builder-mac' after 33 s
[14:29:20] Finished 'pack-mac' after 41 s
~/projects/mullvadvpn-app
**********************************

 The build finished successfully!
 You have built:

 2024.7-dev-1f330c

**********************************

Sorry for the inconvenience 👉 👈 🥺

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This change is 

[faern]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved