Fix autostart filenames on Linux
Summary:
Autostart filenames on Linux must end with .desktop, not .lnk.
Backport of Core PR16379
Test Plan:
ninja && src/qt/bitcoin-qt -regtest
Menu Settings -> Options -> Main, check Start Bitcoin ABC on system login
ls $HOME/.config/autostart/bitcoin-regtest.desktop
Log-out, log back in, verify that bitcoin-qt -regtest is started.
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7885