fix: improper syntax for .env (#523)

* fix: improper syntax for .env

sigh

* remove platforms env var

honestly it's useless now that the other action doesn't have it

.github/workflows/docker-build.yaml

@@ -9,9 +9,6 @@ on:
     paths-ignore: ["**.md", "ext/**"]
   workflow_dispatch:
 
-env:
-  PLATFORMS: linux/amd64
-
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -52,7 +49,7 @@ jobs:
         if: ${{ github.ref == 'refs/heads/master' }}
         with:
           context: .
-          platforms: $PLATFORMS
+          platforms: linux/amd64
           push: true
           tags: ${{ secrets.DOCKERHUB_USERNAME }}/bancho.py:latest
           cache-from: type=gha