fix(ci): repair the workflow YAML broken by an unindented heredoc

The previous commit made build.yml unparseable and no job ran at all:

  yaml: line 183: could not find expected ':'

A `run: |` block is a YAML literal scalar, so every line has to keep the
block's indentation. The python3 fallback I added used a heredoc whose
body sat at column 0, which terminates the scalar -- YAML then tried to
read `import zipfile, sys` as a mapping and gave up. Worse than a broken
check: a broken pipeline.

The fallback is gone rather than re-indented. unzip is present on this
runner, a second code path existed only to guard against a case that was
never observed, and its only contribution was an escaping hazard inside
YAML inside shell.

Verified before pushing this time, which is the actual lesson:
- build.yml now parses (yaml.safe_load), 6 + 15 steps;
- the guard's `run` script was extracted from the parsed YAML and executed
  verbatim against the real 45MB base.apk pulled off the device. It prints
  the drawable inventory, reports ic_stat_pluriwave OK and ic_auto_eq_on /
  ic_auto_eq_off missing, and exits 1 -- matching an independent zipfile
  inspection of the same file.

Two commits in a row shipped a CI change that had never been run. Both
were caught by the user rather than by me.

.gitea/workflows/build.yml

@@ -176,19 +176,11 @@ jobs:
           fi
           echo "APK: $APK ($(wc -c < "$APK") bytes)"
 
-          if command -v unzip >/dev/null 2>&1; then
-            LISTADO=$(unzip -Z1 "$APK")
-          elif command -v python3 >/dev/null 2>&1; then
-            LISTADO=$(python3 - "$APK" <<'PYZIP'
-import zipfile, sys
-for nombre in zipfile.ZipFile(sys.argv[1]).namelist():
-    print(nombre)
-PYZIP
-            )
-          else
-            echo "Ni unzip ni python3 disponibles: no se puede inspeccionar el APK."
+          if ! command -v unzip >/dev/null 2>&1; then
+            echo "unzip no esta disponible: no se puede inspeccionar el APK."
             exit 1
           fi
+          LISTADO=$(unzip -Z1 "$APK")
 
           if [ -z "$LISTADO" ]; then
             echo "El listado del APK salio vacio; la inspeccion no es fiable."