CI: ignore parse errors in gcovr

`gcovr 7.0` used by CI has a bug: it cannot parse line numbers \>9999. 

One of our source files (rdopt.c) has 12K+ lines, so is affected by this bug. 

We avoid this bug by ignoring parsing erros in `gcovr`.

Fixes coverage report generation and closes #756

.gitlab/ci_nightly.yml

diff --git a/.gitlab/ci_nightly.yml b/.gitlab/ci_nightly.yml
index 9e9d37b..2a5854b 100644
--- a/.gitlab/ci_nightly.yml
+++ b/.gitlab/ci_nightly.yml
@@ -53,6 +53,7 @@
     - ./test_libaom
     - >-
       gcovr
+      --gcov-ignore-parse-errors
       --gcov-executable gcov-14
       --exclude ../third_party --exclude _deps --exclude abseil-cpp --exclude
       benchmark --exclude cpuinfo --exclude eigen --exclude farmhash --exclude