ci: run build-images' shell steps under sh, not bash

docker:27-cli (Alpine) has no bash, only the POSIX /bin/sh (busybox
ash). run: steps default to bash, so both steps in build-images failed
immediately with "exec: bash: executable file not found in $PATH" -
confirmed via the run's log right after the previous checkout fix
landed. Both steps are already plain POSIX shell, so declare shell: sh
as the job default instead of switching interpreters.

.gitea/workflows/coder-templates.yml

@@ -51,6 +51,13 @@ jobs:
     runs-on: docker-build
     outputs:
       failed_templates: ${{ steps.build.outputs.failed_templates }}
+    # docker:27-cli (Alpine) has no bash - only the POSIX /bin/sh (busybox
+    # ash) - but run: steps default to bash, which fails with "exec: bash:
+    # executable file not found in $PATH". Both run: steps below are plain
+    # POSIX shell already, so just run them under sh.
+    defaults:
+      run:
+        shell: sh
     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