[ci] Pin Discord webhook action to specific commit sha (facebook#32649)

Pins the discord webhook action to
`86dc739f3f165f16dadc5666051c367efa1692f4`, which is what the v6 tag
points to.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32649).
* facebook#32650
* __->__ facebook#32649
* facebook#32648

Author: poteto

.github/workflows/compiler_discord_notify.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Discord Webhook Action
-        uses: tsickert/discord-webhook@v6.0.0
+        uses: tsickert/discord-webhook@86dc739f3f165f16dadc5666051c367efa1692f4
         with:
           webhook-url: ${{ secrets.COMPILER_DISCORD_WEBHOOK_URL }}
           embed-author-name: ${{ github.event.pull_request.user.login }}

.github/workflows/runtime_discord_notify.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Discord Webhook Action
-        uses: tsickert/discord-webhook@v6.0.0
+        uses: tsickert/discord-webhook@86dc739f3f165f16dadc5666051c367efa1692f4
         with:
           webhook-url: ${{ secrets.DISCORD_WEBHOOK_URL }}
           embed-author-name: ${{ github.event.pull_request.user.login }}

.github/workflows/runtime_releases_from_npm_manual.yml

@@ -44,7 +44,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Discord Webhook Action
-        uses: tsickert/discord-webhook@v6.0.0
+        uses: tsickert/discord-webhook@86dc739f3f165f16dadc5666051c367efa1692f4
         with:
           webhook-url: ${{ secrets.DISCORD_WEBHOOK_URL }}
           embed-author-name: ${{ github.event.sender.login }}