ci: replace actions/checkout with a raw git clone in build-images

actions/checkout is a JS action and needs Node to run inside the job
container. The docker-build runner's job image (docker:27-cli, Alpine,
just the Docker CLI) has no Node, so the step failed immediately with
"exec: node: executable file not found in $PATH" - confirmed via the
run's actual log (build-images failed after 16s on every push since the
image-in-CI switch, including the just-merged duplicate-mount and
if:always() fixes). Alpine has apk/git, so clone directly instead.

.gitea/workflows/coder-templates.yml

@@ -52,8 +52,16 @@ jobs:
     outputs:
       failed_templates: ${{ steps.build.outputs.failed_templates }}
     steps:
+      # actions/checkout is a JS action and needs Node in the job container -
+      # this job runs in docker:27-cli (Alpine, just the Docker CLI) so it has
+      # no Node, and actions/checkout fails immediately with "exec: node:
+      # executable file not found in $PATH". Alpine does have apk/git though,
+      # so clone directly instead.
       - name: Checkout
-        uses: actions/checkout@v4
+        run: |
+          set -e
+          apk add --no-cache git
+          git clone --depth 1 --branch "${{ github.ref_name }}" "${{ github.server_url }}/${{ github.repository }}.git" .
 
       - name: Build and push every template's image (skips a tag that's already in the registry)
         id: build