Use openFolder on Windows and Linux (fixes #18859)

src/vs/workbench/parts/welcomePage/electron-browser/welcomePage.css

@@ -230,3 +230,18 @@
 .monaco-workbench > .part.editor > .content .welcomePage.max-height-690px .title {
 	display: none;
 }
+
+.monaco-workbench > .part.editor > .content .welcomePage .mac-only,
+.monaco-workbench > .part.editor > .content .welcomePage .windows-only,
+.monaco-workbench > .part.editor > .content .welcomePage .linux-only {
+	display: none;
+}
+.monaco-workbench.mac > .part.editor > .content .welcomePage .mac-only {
+	display: initial;
+}
+.monaco-workbench.windows > .part.editor > .content .welcomePage .windows-only {
+	display: initial;
+}
+.monaco-workbench.linux > .part.editor > .content .welcomePage .linux-only {
+	display: initial;
+}
\ No newline at end of file

src/vs/workbench/parts/welcomePage/electron-browser/welcomePage.html

@@ -8,7 +8,8 @@
 			<div class="section start">
 				<h2>Start</h2>
 				<ul>
-					<li><a href="command:workbench.action.files.openFileFolder">Open folder...</a></li>
+					<li class="mac-only"><a href="command:workbench.action.files.openFileFolder">Open folder...</a></li>
+					<li class="windows-only linux-only"><a href="command:workbench.action.files.openFolder">Open folder...</a></li>
 					<li><a href="command:workbench.action.git.clone">Clone Git repository...</a></li>
 				</ul>
 			</div>