ci: Fix check for missing install tag

Better check the installation plan json file instead of parsing logs.

.gitlab-ci/check-missing-install-tag.py

@@ -6,6 +6,7 @@ missing installation tag.
 """
 
 import argparse
+import json
 from pathlib import Path
 
 
@@ -14,14 +15,16 @@ def main():
     parser.add_argument("builddir", type=Path)
     args = parser.parse_args()
 
-    logfile = args.builddir / "meson-logs" / "meson-log.txt"
-    with logfile.open(encoding="utf-8") as f:
-        if "Failed to guess install tag" in f.read():
-            print(
-                f"Some files are missing install_tag, see {logfile} for details."  # no-qa
-            )
-            return 1
-    return 0
+    success = True
+    path = args.builddir / "meson-info" / "intro-install_plan.json"
+    with path.open(encoding="utf-8") as f:
+        install_plan = json.load(f)
+        for target in install_plan.values():
+            for info in target.values():
+                if not info["tag"]:
+                    print('Missing install_tag for', info["destination"])
+                    success = False
+    return 0 if success else 1
 
 
 if __name__ == "__main__":