Add x-scheme-handler to the desktop file script

[wiebel]

Summary

Small insertion to the Mattermost.desktop file PR as requested
here

Ticket Link

Fixes part of #2906

Release Note

NONE

[mm-cloud-bot]

@wiebel: Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[mattermost-build]

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[devinbinnie]

@wiebel Change looks good, thanks for doing that :)
Can you make sure to sign the CLA as per above? Then I can finish reviewing.

[wiebel]

/check-cla

[devinbinnie]

LGTM! Thanks @wiebel!