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

The test is broken by 1.57.0-insider - the vscode fails to get
initialized before activating extension under testing.
Unblock nightly release while investigating the upstream issue
(microsoft/vscode#125556)

Updates #1543

Change-Id: If1b76190be6ea81eabc30c7a79502282f25871eb
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/325579
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

Author: hyangah

.github/workflows/release-nightly.yml

@@ -1,5 +1,7 @@
 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" }'
@@ -79,7 +81,6 @@ jobs:
         with:
           run: npm run test
         env:
-          CODE_VERSION: 'insiders'
           VSCODEGO_BEFORE_RELEASE_TESTS: true
 
       - name: Publish