harness-base: fix python3 -c quoting inside nix string

Two consecutive single quotes ('') inside a Nix indented string (''...'')
are treated as the string-end delimiter, breaking nix evaluation.
Switch the shell -c argument from double-quotes to single-quotes so
the Python string literals use double-quotes instead, avoiding any ''
sequences in the Nix source.

nix/templates/harness-base.nix

@@ -317,7 +317,7 @@
         USER=$(curl -sf --max-time 5 \
           -H "Authorization: token $TOKEN" \
           "$FORGE_URL/api/v1/user" \
-          | python3 -c "import sys,json; print(json.load(sys.stdin).get('login',''))" \
+          | python3 -c 'import sys,json; print(json.load(sys.stdin).get("login",""))' \
           2>/dev/null || true)
         if [ -z "$USER" ]; then
           echo "tea-login: could not resolve username from forge API; skipping"