twister: coverage: initialize 'use_system_gcov'

Initialize 'use_system_gcov', so it is always set before being referenced
in "elif" statement.

Fixes: 1440b19bde ("twister: restore logic to determine which gcov we
  should use")
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

scripts/pylib/twister/twisterlib/coverage.py

@@ -237,6 +237,8 @@ class Gcovr(CoverageTool):
 
 
 def run_coverage(testplan, options):
+    use_system_gcov = False
+
     for plat in options.coverage_platform:
         _plat = testplan.get_platform(plat)
         if _plat and (_plat.type in {"native", "unit"}):