fix: indent heredoc content to satisfy YAML block scalar parser

YAML ends a block scalar when it sees content less indented than
the first content line. The JSON heredoc at column 0 caused the
parser to bail out mid-block. Indenting to 8 spaces keeps it inside
the run: | scalar; YAML strips that indentation before handing the
script to the shell, so the NOMAD_EOF terminator lands at column 0
as required.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

deploy-nix-site/action.yaml

@@ -79,7 +79,7 @@ runs:
       shell: bash
       run: |
         cat > /tmp/deploy-${{ inputs.site-name }}.nomad.json <<'NOMAD_EOF'
-{
+        {
           "Job": {
             "ID": "${{ inputs.site-name }}",
             "Name": "${{ inputs.site-name }}",
@@ -172,8 +172,8 @@ runs:
               ]
             }]
           }
-}
-NOMAD_EOF
+        }
+        NOMAD_EOF
 
         nomad job run -json /tmp/deploy-${{ inputs.site-name }}.nomad.json