src/goMain: fix build breakage - isNightly is renamed

Change-Id: Ic82b951268e8509eb25f95e4980bfdedf3151b04
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/283935
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
diff --git a/src/goMain.ts b/src/goMain.ts
index 98d08bc..0824136 100644
--- a/src/goMain.ts
+++ b/src/goMain.ts
@@ -573,7 +573,7 @@
 	const showVersions: string[] = [];
 
 	let goExtensionVersionKey = 'go.extensionVersion';
-	if (isNightly()) {
+	if (isInPreviewMode()) {
 		goExtensionVersionKey = 'go.nightlyExtensionVersion';
 	}