Merge #16379: Fix autostart filenames on Linux for testnet/regtest

ae311bc Fix autostart filenames on Linux (Hennadii Stepanov)

Pull request description:

  Currently, on master the `bitcoin-test.lnk` and `bitcoin-regtest.lnk` files do not work as autostart application `.desktop` files.

  This PR fixes it.

  Refs:
  - #7045
  - [Autostart Of Applications During Startup](https://standards.freedesktop.org/autostart-spec/autostart-spec-latest.html)

ACKs for top commit:
  promag:
    utACK ae311bc, weird why extension `.lnk` was used in #7045.
  laanwj:
    Code review ACK ae311bc

Tree-SHA512: 210cc346600d52b0a262c81ed5f258365a3cea2e5522f4b5f4798fd3b54f45ed82aba68eefae59a6b6f1d8e4d00221476c23bdffc038f16f2f45c1acc837f522

Author: laanwj

src/qt/guiutil.cpp

@@ -639,7 +639,7 @@ fs::path static GetAutostartFilePath()
     std::string chain = gArgs.GetChainName();
     if (chain == CBaseChainParams::MAIN)
         return GetAutostartDir() / "bitcoin.desktop";
-    return GetAutostartDir() / strprintf("bitcoin-%s.lnk", chain);
+    return GetAutostartDir() / strprintf("bitcoin-%s.desktop", chain);
 }
 
 bool GetStartOnSystemStartup()