workflows: remove use-feature=in-tree-build

Using `--use-feature=in-tree-build` is no longer needed for recent pip versions,
doing so raised:

```
option --use-feature: invalid choice: 'in-tree-build' (choose from 'fast-deps', 'truststore', 'no-binary-enable-wheel-cache')
```

.github/workflows/main.yml

@@ -69,7 +69,7 @@ jobs:
           pytest-custom-exit-code==0.3.0 \
           pytest-json-report
 
-        pip install --user -U -r requirements.lock  --use-feature=in-tree-build
+        pip install --user -U -r requirements.lock
 
         ./configure --disable-valgrind
         make -j 16