Stop using an environment variable to store platforms

Instead of using the CSV style in an environment variable we can
instead use a newline delimited list directly for the `platforms`
input. Since the environment variable is only used to provide a value
for the `platforms` input there should be no issue with this change.

.github/workflows/build.yml

@@ -40,8 +40,6 @@ env:
   BUILDX_CACHE_DIR: ~/.cache/buildx
   IMAGE_NAME: cisagov/example
   PIP_CACHE_DIR: ~/.cache/pip
-  PLATFORMS: "linux/amd64,linux/arm/v6,linux/arm/v7,\
-  linux/arm64,linux/ppc64le,linux/s390x"
   PRE_COMMIT_CACHE_DIR: ~/.cache/pre-commit
   RUN_TMATE: ${{ secrets.RUN_TMATE }}
   TERRAFORM_DOCS_REPO_BRANCH_NAME: improvement/support_atx_closed_markdown_headers
@@ -452,12 +450,12 @@ jobs:
         uses: mxschmitt/action-tmate@v3
         if: env.RUN_TMATE
   build-push-all:
-    # Builds the final set of images for each of the platforms listed in
-    # PLATFORMS environment variable.  These images are tagged with the Docker
-    # tags calculated in the "prepare" job and pushed to Docker Hub and the
-    # GitHub Container Registry.  The contents of README.md are pushed as the
-    # image's description to Docker Hub.  This job is skipped when the
-    # triggering event is a pull request.
+    # Builds the final set of images for each of the platforms specified in the
+    # "platforms" input for the docker/build-push-action Action.  These images
+    # are tagged with the Docker tags calculated in the "prepare" job and
+    # pushed to Docker Hub and the GitHub Container Registry.  The contents of
+    # README.md are pushed as the image's description to Docker Hub.  This job
+    # is skipped when the triggering event is a pull request.
     if: github.event_name != 'pull_request'
     name: Build and push all platforms
     needs:
@@ -541,7 +539,13 @@ jobs:
 
             org.opencontainers.image.version=${{
               needs.prepare.outputs.source_version }}"
-          platforms: ${{ env.PLATFORMS }}
+          platforms: |
+            linux/amd64
+            linux/arm/v6
+            linux/arm/v7
+            linux/arm64
+            linux/ppc64le
+            linux/s390x
           # Uncomment the following option if you are building an image for use
           # on Google Cloud Run or AWS Lambda. The current default image output
           # is unable to run on either. Please see the following issue for more