Revert ".github/workflows: use 'stable' version for release-nightly"

This reverts commit 936ecdf.

Reason for revert: microsoft/vscode#125556 is likely fixed with the newer insiders.

Updates #1543

Change-Id: I3b6d715c59dd2d78d6037c85ab4b82de97f24e5d
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/325929
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>

Author: hyangah

.github/workflows/release-nightly.yml

@@ -1,7 +1,5 @@
 name: Release (golang.go-nightly)
 
-# TODO(hyangah): Reenable to use 'insiders' after golang/vscode-go#1543 is fixed.
-
 # Daily release on 15:00 UTC, monday-thursday.
 # Or, force to release by triggering repository_dispatch events by using
 #   curl -v -H "Accept: application/vnd.github.everest-preview+json" -H "Authorization: token ${GITHUB_TOKEN}" https://api.github.com/repos/golang/vscode-go/dispatches -d '{ "event_type": "force-release" }'
@@ -81,6 +79,7 @@ jobs:
         with:
           run: npm run test
         env:
+          CODE_VERSION: 'insiders'
           VSCODEGO_BEFORE_RELEASE_TESTS: true
 
       - name: Publish