ci: of course also install igraph itself

Author: ntamas

.github/workflows/build.yml

@@ -338,6 +338,7 @@ jobs:
           # We cannot install the test dependency group because many test dependencies cause
           # false positives in the sanitizer
           pip install --prefer-binary networkx pytest pytest-timeout
+          pip install -e .
 
       # Only pytest, and nothing else should be run in this section due to the presence of LD_PRELOAD.
       # The ASan/UBSan library versions need to be updated when switching to a newer Ubuntu/GCC.