CI: Re-enable code coverage reporting for MSYS2 builds

It was disabled in !875 because lcov didn't support the new coverage
format produced by gcc9+. The latest lcov release in MSYS2 supports
it again, so re-enable everything.

lcov now writes native Windows paths to its output so adjust the path
fixup script to handle those.

.gitlab-ci/fixup-cov-paths.py

@@ -1,6 +1,7 @@
 import sys
 import os
 import io
+import re
 
 
 def main(argv):
@@ -12,11 +13,7 @@ def main(argv):
         print("cov-fixup:", path)
         text = io.open(path, "r", encoding="utf-8").read()
         text = text.replace("\\\\", "/")
-        glib_dir = "/glib/"
-        end = text.index(glib_dir)
-        start = text[:end].rindex(":") + 1
-        old_root = text[start:end]
-        assert os.path.basename(os.getcwd()) == "glib"
+        old_root = re.search(":(.*?)/glib/.*?$", text, re.MULTILINE).group(1)
         new_root = os.path.dirname(os.getcwd())
         if old_root != new_root:
             print("replacing %r with %r" % (old_root, new_root))